diff --git a/lib/includes/nghttp2/nghttp2.h b/lib/includes/nghttp2/nghttp2.h
index dc1aa301..52cc0a0a 100644
--- a/lib/includes/nghttp2/nghttp2.h
+++ b/lib/includes/nghttp2/nghttp2.h
@@ -845,7 +845,7 @@ typedef struct {
   /**
    * The error code.  See :type:`nghttp2_error_code`.
    */
-  nghttp2_error_code error_code;
+  uint32_t error_code;
 } nghttp2_rst_stream;
 
 /**
@@ -946,7 +946,7 @@ typedef struct {
   /**
    * The error code.  See :type:`nghttp2_error_code`.
    */
-  nghttp2_error_code error_code;
+  uint32_t error_code;
   /**
    * The additional debug data
    */
@@ -1178,12 +1178,13 @@ typedef int (*nghttp2_on_frame_recv_callback)
  * @functypedef
  *
  * Callback function invoked by `nghttp2_session_recv()` when an
- * invalid non-DATA frame is received.  The |error_code| is one of the
- * :enum:`nghttp2_error_code` and indicates the error.  When this
- * callback function is invoked, the library automatically submits
- * either RST_STREAM or GOAWAY frame.  The |user_data| pointer is the
- * third argument passed in to the call to
- * `nghttp2_session_client_new()` or `nghttp2_session_server_new()`.
+ * invalid non-DATA frame is received.  The |error_code| indicates the
+ * error.  It is usually one of the :enum:`nghttp2_error_code` but
+ * that is not guaranteed.  When this callback function is invoked,
+ * the library automatically submits either RST_STREAM or GOAWAY
+ * frame.  The |user_data| pointer is the third argument passed in to
+ * the call to `nghttp2_session_client_new()` or
+ * `nghttp2_session_server_new()`.
  *
  * If frame is HEADERS or PUSH_PROMISE, the ``nva`` and ``nvlen``
  * member of their data structure are always ``NULL`` and 0
@@ -1199,7 +1200,7 @@ typedef int (*nghttp2_on_frame_recv_callback)
  */
 typedef int (*nghttp2_on_invalid_frame_recv_callback)
 (nghttp2_session *session, const nghttp2_frame *frame,
- nghttp2_error_code error_code, void *user_data);
+ uint32_t error_code, void *user_data);
 
 /**
  * @functypedef
@@ -1299,10 +1300,11 @@ typedef int (*nghttp2_on_frame_not_send_callback)
  *
  * Callback function invoked when the stream |stream_id| is closed.
  * The reason of closure is indicated by the |error_code|.  The
- * stream_user_data, which was specified in `nghttp2_submit_request()`
- * or `nghttp2_submit_headers()`, is still available in this function.
- * The |user_data| pointer is the third argument passed in to the call
- * to `nghttp2_session_client_new()` or
+ * |error_code| is usually one of :enum:`nghttp2_error_code`, but that
+ * is not guaranteed.  The stream_user_data, which was specified in
+ * `nghttp2_submit_request()` or `nghttp2_submit_headers()`, is still
+ * available in this function.  The |user_data| pointer is the third
+ * argument passed in to the call to `nghttp2_session_client_new()` or
  * `nghttp2_session_server_new()`.
  *
  * This function is also called for a stream in reserved state.
@@ -1316,7 +1318,7 @@ typedef int (*nghttp2_on_frame_not_send_callback)
  * `nghttp2_session_callbacks_set_on_stream_close_callback()`.
  */
 typedef int (*nghttp2_on_stream_close_callback)
-(nghttp2_session *session, int32_t stream_id, nghttp2_error_code error_code,
+(nghttp2_session *session, int32_t stream_id, uint32_t error_code,
  void *user_data);
 
 /**
@@ -2173,7 +2175,8 @@ int nghttp2_session_get_stream_remote_close(nghttp2_session* session,
  * The last stream ID is the ID of a stream for which
  * :type:`nghttp2_on_frame_recv_callback` was called most recently.
  *
- * The |error_code| is the error code of this GOAWAY frame.
+ * The |error_code| is the error code of this GOAWAY frame.  The
+ * pre-defined error code is one of :enum:`nghttp2_error_code`.
  *
  * After the transmission, both `nghttp2_session_want_read()` and
  * `nghttp2_session_want_write()` return 0.
@@ -2189,7 +2192,7 @@ int nghttp2_session_get_stream_remote_close(nghttp2_session* session,
  *     Out of memory.
  */
 int nghttp2_session_terminate_session(nghttp2_session *session,
-                                      nghttp2_error_code error_code);
+                                      uint32_t error_code);
 
 /**
  * @function
@@ -2208,7 +2211,7 @@ int nghttp2_session_terminate_session(nghttp2_session *session,
  */
 int nghttp2_session_terminate_session2(nghttp2_session *session,
                                        int32_t last_stream_id,
-                                       nghttp2_error_code error_code);
+                                       uint32_t error_code);
 
 /**
  * @function
@@ -2603,6 +2606,8 @@ int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags,
  * Submits RST_STREAM frame to cancel/reject the stream |stream_id|
  * with the error code |error_code|.
  *
+ * The pre-defined error code is one of :enum:`nghttp2_error_code`.
+ *
  * The |flags| is currently ignored and should be
  * :enum:`NGHTTP2_FLAG_NONE`.
  *
@@ -2616,7 +2621,7 @@ int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags,
  */
 int nghttp2_submit_rst_stream(nghttp2_session *session, uint8_t flags,
                               int32_t stream_id,
-                              nghttp2_error_code error_code);
+                              uint32_t error_code);
 
 /**
  * @function
@@ -2743,6 +2748,8 @@ int nghttp2_submit_ping(nghttp2_session *session, uint8_t flags,
  * Submits GOAWAY frame with the last stream ID |last_stream_id| and
  * the error code |error_code|.
  *
+ * The pre-defined error code is one of :enum:`nghttp2_error_code`.
+ *
  * The |flags| is currently ignored and should be
  * :enum:`NGHTTP2_FLAG_NONE`.
  *
@@ -2769,7 +2776,7 @@ int nghttp2_submit_ping(nghttp2_session *session, uint8_t flags,
  */
 int nghttp2_submit_goaway(nghttp2_session *session, uint8_t flags,
                           int32_t last_stream_id,
-                          nghttp2_error_code error_code,
+                          uint32_t error_code,
                           const uint8_t *opaque_data, size_t opaque_data_len);
 
 /**
diff --git a/lib/nghttp2_frame.c b/lib/nghttp2_frame.c
index 228284f7..b351e4f3 100644
--- a/lib/nghttp2_frame.c
+++ b/lib/nghttp2_frame.c
@@ -101,7 +101,7 @@ void nghttp2_frame_priority_free(nghttp2_priority *frame)
 
 void nghttp2_frame_rst_stream_init(nghttp2_rst_stream *frame,
                                    int32_t stream_id,
-                                   nghttp2_error_code error_code)
+                                   uint32_t error_code)
 {
   frame_set_hd(&frame->hd, 4, NGHTTP2_RST_STREAM, NGHTTP2_FLAG_NONE,
                stream_id);
@@ -158,7 +158,7 @@ void nghttp2_frame_ping_free(nghttp2_ping *frame)
 {}
 
 void nghttp2_frame_goaway_init(nghttp2_goaway *frame, int32_t last_stream_id,
-                               nghttp2_error_code error_code,
+                               uint32_t error_code,
                                uint8_t *opaque_data, size_t opaque_data_len)
 {
   frame_set_hd(&frame->hd, 8+opaque_data_len, NGHTTP2_GOAWAY,
@@ -477,33 +477,11 @@ int nghttp2_frame_pack_rst_stream(nghttp2_bufs *bufs,
   return 0;
 }
 
-static nghttp2_error_code normalize_error_code(uint32_t error_code)
-{
-  switch(error_code) {
-  case NGHTTP2_NO_ERROR:
-  case NGHTTP2_PROTOCOL_ERROR:
-  case NGHTTP2_INTERNAL_ERROR:
-  case NGHTTP2_FLOW_CONTROL_ERROR:
-  case NGHTTP2_SETTINGS_TIMEOUT:
-  case NGHTTP2_STREAM_CLOSED:
-  case NGHTTP2_FRAME_SIZE_ERROR:
-  case NGHTTP2_REFUSED_STREAM:
-  case NGHTTP2_CANCEL:
-  case NGHTTP2_COMPRESSION_ERROR:
-  case NGHTTP2_CONNECT_ERROR:
-  case NGHTTP2_ENHANCE_YOUR_CALM:
-  case NGHTTP2_INADEQUATE_SECURITY:
-    return error_code;
-  default:
-    return NGHTTP2_INTERNAL_ERROR;
-  }
-}
-
 void nghttp2_frame_unpack_rst_stream_payload(nghttp2_rst_stream *frame,
                                              const uint8_t *payload,
                                              size_t payloadlen)
 {
-  frame->error_code = normalize_error_code(nghttp2_get_uint32(payload));
+  frame->error_code = nghttp2_get_uint32(payload);
 }
 
 int nghttp2_frame_pack_settings(nghttp2_bufs *bufs, nghttp2_settings *frame)
@@ -712,7 +690,7 @@ void nghttp2_frame_unpack_goaway_payload(nghttp2_goaway *frame,
                                          size_t var_gift_payloadlen)
 {
   frame->last_stream_id = nghttp2_get_uint32(payload) & NGHTTP2_STREAM_ID_MASK;
-  frame->error_code = normalize_error_code(nghttp2_get_uint32(payload + 4));
+  frame->error_code = nghttp2_get_uint32(payload + 4);
 
   frame->opaque_data = var_gift_payload;
   frame->opaque_data_len = var_gift_payloadlen;
diff --git a/lib/nghttp2_frame.h b/lib/nghttp2_frame.h
index 6fa86af8..b0a04438 100644
--- a/lib/nghttp2_frame.h
+++ b/lib/nghttp2_frame.h
@@ -477,7 +477,7 @@ void nghttp2_frame_priority_free(nghttp2_priority *frame);
 
 void nghttp2_frame_rst_stream_init(nghttp2_rst_stream *frame,
                                    int32_t stream_id,
-                                   nghttp2_error_code error_code);
+                                   uint32_t error_code);
 
 void nghttp2_frame_rst_stream_free(nghttp2_rst_stream *frame);
 
@@ -519,7 +519,7 @@ void nghttp2_frame_ping_free(nghttp2_ping *frame);
  * free it. If the |opaque_data_len| is 0, opaque_data could be NULL.
  */
 void nghttp2_frame_goaway_init(nghttp2_goaway *frame, int32_t last_stream_id,
-                               nghttp2_error_code error_code,
+                               uint32_t error_code,
                                uint8_t *opaque_data, size_t opaque_data_len);
 
 void nghttp2_frame_goaway_free(nghttp2_goaway *frame);
diff --git a/lib/nghttp2_session.c b/lib/nghttp2_session.c
index 53dd8485..835cb00d 100644
--- a/lib/nghttp2_session.c
+++ b/lib/nghttp2_session.c
@@ -134,7 +134,7 @@ static int session_detect_idle_stream(nghttp2_session *session,
 
 static int session_terminate_session
 (nghttp2_session *session, int32_t last_stream_id,
- nghttp2_error_code error_code, const char *reason)
+ uint32_t error_code, const char *reason)
 {
   const uint8_t *debug_data;
   size_t debug_datalen;
@@ -157,7 +157,7 @@ static int session_terminate_session
 }
 
 int nghttp2_session_terminate_session(nghttp2_session *session,
-                                      nghttp2_error_code error_code)
+                                      uint32_t error_code)
 {
   return session_terminate_session(session, session->last_proc_stream_id,
                                    error_code, NULL);
@@ -165,13 +165,13 @@ int nghttp2_session_terminate_session(nghttp2_session *session,
 
 int nghttp2_session_terminate_session2(nghttp2_session *session,
                                        int32_t last_stream_id,
-                                       nghttp2_error_code error_code)
+                                       uint32_t error_code)
 {
   return session_terminate_session(session, last_stream_id, error_code, NULL);
 }
 
 int nghttp2_session_terminate_session_with_reason
-(nghttp2_session *session, nghttp2_error_code error_code, const char *reason)
+(nghttp2_session *session, uint32_t error_code, const char *reason)
 {
   return session_terminate_session(session, session->last_proc_stream_id,
                                    error_code, reason);
@@ -707,7 +707,7 @@ int nghttp2_session_add_frame(nghttp2_session *session,
 
 int nghttp2_session_add_rst_stream(nghttp2_session *session,
                                    int32_t stream_id,
-                                   nghttp2_error_code error_code)
+                                   uint32_t error_code)
 {
   int rv;
   nghttp2_frame *frame;
@@ -832,7 +832,7 @@ nghttp2_stream* nghttp2_session_open_stream(nghttp2_session *session,
 }
 
 int nghttp2_session_close_stream(nghttp2_session *session, int32_t stream_id,
-                                 nghttp2_error_code error_code)
+                                 uint32_t error_code)
 {
   int rv;
   nghttp2_stream *stream;
@@ -2525,7 +2525,7 @@ static int session_handle_frame_size_error(nghttp2_session *session,
 static int session_handle_invalid_stream
 (nghttp2_session *session,
  nghttp2_frame *frame,
- nghttp2_error_code error_code)
+ uint32_t error_code)
 {
   int rv;
   rv = nghttp2_session_add_rst_stream(session, frame->hd.stream_id, error_code);
@@ -2544,7 +2544,7 @@ static int session_handle_invalid_stream
 static int session_inflate_handle_invalid_stream
 (nghttp2_session *session,
  nghttp2_frame *frame,
- nghttp2_error_code error_code)
+ uint32_t error_code)
 {
   int rv;
   rv = session_handle_invalid_stream(session, frame, error_code);
@@ -2560,7 +2560,7 @@ static int session_inflate_handle_invalid_stream
 static int session_handle_invalid_connection
 (nghttp2_session *session,
  nghttp2_frame *frame,
- nghttp2_error_code error_code,
+ uint32_t error_code,
  const char *reason)
 {
   if(session->callbacks.on_invalid_frame_recv_callback) {
@@ -2576,7 +2576,7 @@ static int session_handle_invalid_connection
 static int session_inflate_handle_invalid_connection
 (nghttp2_session *session,
  nghttp2_frame *frame,
- nghttp2_error_code error_code,
+ uint32_t error_code,
  const char *reason)
 {
   int rv;
@@ -3345,7 +3345,7 @@ int nghttp2_session_on_settings_received(nghttp2_session *session,
     session->inflight_iv = NULL;
     session->inflight_niv = -1;
     if(rv != 0) {
-      nghttp2_error_code error_code = NGHTTP2_INTERNAL_ERROR;
+      uint32_t error_code = NGHTTP2_INTERNAL_ERROR;
       if(nghttp2_is_fatal(rv)) {
         return rv;
       }
@@ -5416,7 +5416,7 @@ int nghttp2_session_add_ping(nghttp2_session *session, uint8_t flags,
 
 int nghttp2_session_add_goaway(nghttp2_session *session,
                                int32_t last_stream_id,
-                               nghttp2_error_code error_code,
+                               uint32_t error_code,
                                const uint8_t *opaque_data,
                                size_t opaque_data_len)
 {
diff --git a/lib/nghttp2_session.h b/lib/nghttp2_session.h
index 8c8a13c6..ac60dd96 100644
--- a/lib/nghttp2_session.h
+++ b/lib/nghttp2_session.h
@@ -290,7 +290,7 @@ int nghttp2_session_add_frame(nghttp2_session *session,
  */
 int nghttp2_session_add_rst_stream(nghttp2_session *session,
                                    int32_t stream_id,
-                                   nghttp2_error_code error_code);
+                                   uint32_t error_code);
 
 /*
  * Adds PING frame. This is a convenient functin built on top of
@@ -324,7 +324,7 @@ int nghttp2_session_add_ping(nghttp2_session *session, uint8_t flags,
  */
 int nghttp2_session_add_goaway(nghttp2_session *session,
                                int32_t last_stream_id,
-                               nghttp2_error_code error_code,
+                               uint32_t error_code,
                                const uint8_t *opaque_data,
                                size_t opaque_data_len);
 
@@ -396,7 +396,7 @@ nghttp2_stream* nghttp2_session_open_stream(nghttp2_session *session,
  *     The callback function failed.
  */
 int nghttp2_session_close_stream(nghttp2_session *session, int32_t stream_id,
-                                 nghttp2_error_code error_code);
+                                 uint32_t error_code);
 
 /*
  * Deletes |stream| from memory.  After this function returns, stream
@@ -739,6 +739,6 @@ int nghttp2_session_reprioritize_stream
  *     The |reason| is too long.
  */
 int nghttp2_session_terminate_session_with_reason
-(nghttp2_session *session, nghttp2_error_code error_code, const char *reason);
+(nghttp2_session *session, uint32_t error_code, const char *reason);
 
 #endif /* NGHTTP2_SESSION_H */
diff --git a/lib/nghttp2_submit.c b/lib/nghttp2_submit.c
index c89dcc54..72a516a5 100644
--- a/lib/nghttp2_submit.c
+++ b/lib/nghttp2_submit.c
@@ -234,7 +234,7 @@ int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags,
 
 int nghttp2_submit_rst_stream(nghttp2_session *session, uint8_t flags,
                               int32_t stream_id,
-                              nghttp2_error_code error_code)
+                              uint32_t error_code)
 {
   if(stream_id == 0) {
     return NGHTTP2_ERR_INVALID_ARGUMENT;
@@ -245,7 +245,7 @@ int nghttp2_submit_rst_stream(nghttp2_session *session, uint8_t flags,
 
 int nghttp2_submit_goaway(nghttp2_session *session, uint8_t flags,
                           int32_t last_stream_id,
-                          nghttp2_error_code error_code,
+                          uint32_t error_code,
                           const uint8_t *opaque_data, size_t opaque_data_len)
 {
   return nghttp2_session_add_goaway(session, last_stream_id,
diff --git a/src/HttpServer.cc b/src/HttpServer.cc
index f5c16179..3a569195 100644
--- a/src/HttpServer.cc
+++ b/src/HttpServer.cc
@@ -903,8 +903,7 @@ int Http2Handler::submit_push_promise(Stream *stream,
   return 0;
 }
 
-int Http2Handler::submit_rst_stream(Stream *stream,
-                                    nghttp2_error_code error_code)
+int Http2Handler::submit_rst_stream(Stream *stream, uint32_t error_code)
 {
   remove_stream_read_timeout(stream);
   remove_stream_write_timeout(stream);
@@ -967,7 +966,7 @@ void Http2Handler::remove_settings_timer()
   }
 }
 
-void Http2Handler::terminate_session(nghttp2_error_code error_code)
+void Http2Handler::terminate_session(uint32_t error_code)
 {
   nghttp2_session_terminate_session(session_, error_code);
 }
@@ -1430,7 +1429,7 @@ int on_data_chunk_recv_callback
 
 namespace {
 int on_stream_close_callback
-(nghttp2_session *session, int32_t stream_id, nghttp2_error_code error_code,
+(nghttp2_session *session, int32_t stream_id, uint32_t error_code,
  void *user_data)
 {
   auto hd = static_cast<Http2Handler*>(user_data);
diff --git a/src/HttpServer.h b/src/HttpServer.h
index 23048c92..d7e3fa70 100644
--- a/src/HttpServer.h
+++ b/src/HttpServer.h
@@ -130,7 +130,7 @@ public:
 
   int submit_push_promise(Stream *stream, const std::string& push_path);
 
-  int submit_rst_stream(Stream *stream, nghttp2_error_code error_code);
+  int submit_rst_stream(Stream *stream, uint32_t error_code);
 
   void add_stream(int32_t stream_id, std::unique_ptr<Stream> stream);
   void remove_stream(int32_t stream_id);
@@ -141,7 +141,7 @@ public:
   size_t get_left_connhd_len() const;
   void set_left_connhd_len(size_t left);
   void remove_settings_timer();
-  void terminate_session(nghttp2_error_code error_code);
+  void terminate_session(uint32_t error_code);
   int tls_handshake();
 private:
   int handle_ssl_temporal_error(int err);
diff --git a/src/app_helper.cc b/src/app_helper.cc
index cfdc5820..21697b00 100644
--- a/src/app_helper.cc
+++ b/src/app_helper.cc
@@ -52,7 +52,7 @@
 namespace nghttp2 {
 
 namespace {
-const char* strstatus(nghttp2_error_code error_code)
+const char* strstatus(uint32_t error_code)
 {
   switch(error_code) {
   case NGHTTP2_NO_ERROR:
@@ -464,7 +464,7 @@ int verbose_on_frame_recv_callback
 
 int verbose_on_invalid_frame_recv_callback
 (nghttp2_session *session, const nghttp2_frame *frame,
- nghttp2_error_code error_code, void *user_data)
+ uint32_t error_code, void *user_data)
 {
   print_timer();
   fprintf(outfile, " [INVALID; status=%s] recv ", strstatus(error_code));
diff --git a/src/app_helper.h b/src/app_helper.h
index 269ddc7f..4d547e3d 100644
--- a/src/app_helper.h
+++ b/src/app_helper.h
@@ -51,7 +51,7 @@ int verbose_on_frame_recv_callback
 
 int verbose_on_invalid_frame_recv_callback
 (nghttp2_session *session, const nghttp2_frame *frame,
- nghttp2_error_code error_code, void *user_data);
+ uint32_t error_code, void *user_data);
 
 int verbose_on_unknown_frame_recv_callback(nghttp2_session *session,
                                            const uint8_t *head,
diff --git a/src/h2load_http2_session.cc b/src/h2load_http2_session.cc
index 5acdb82a..9cca835f 100644
--- a/src/h2load_http2_session.cc
+++ b/src/h2load_http2_session.cc
@@ -89,7 +89,7 @@ int on_data_chunk_recv_callback
 
 namespace {
 int on_stream_close_callback
-(nghttp2_session *session, int32_t stream_id, nghttp2_error_code error_code,
+(nghttp2_session *session, int32_t stream_id, uint32_t error_code,
  void *user_data)
 {
   auto client = static_cast<Client*>(user_data);
diff --git a/src/nghttp.cc b/src/nghttp.cc
index 06335ce3..651effe1 100644
--- a/src/nghttp.cc
+++ b/src/nghttp.cc
@@ -1474,7 +1474,7 @@ int on_frame_recv_callback2
 
 namespace {
 int on_stream_close_callback
-(nghttp2_session *session, int32_t stream_id, nghttp2_error_code error_code,
+(nghttp2_session *session, int32_t stream_id, uint32_t error_code,
  void *user_data)
 {
   auto client = get_session(user_data);
diff --git a/src/shrpx_downstream.cc b/src/shrpx_downstream.cc
index 4a5d0bc4..f781ee82 100644
--- a/src/shrpx_downstream.cc
+++ b/src/shrpx_downstream.cc
@@ -54,7 +54,7 @@ Downstream::Downstream(Upstream *upstream, int32_t stream_id, int32_t priority)
     stream_id_(stream_id),
     priority_(priority),
     downstream_stream_id_(-1),
-    response_rst_stream_error_code_(-1),
+    response_rst_stream_error_code_(NGHTTP2_NO_ERROR),
     request_state_(INITIAL),
     request_major_(1),
     request_minor_(1),
@@ -848,12 +848,12 @@ int32_t Downstream::get_downstream_stream_id() const
   return downstream_stream_id_;
 }
 
-int Downstream::get_response_rst_stream_error_code() const
+uint32_t Downstream::get_response_rst_stream_error_code() const
 {
   return response_rst_stream_error_code_;
 }
 
-void Downstream::set_response_rst_stream_error_code(int error_code)
+void Downstream::set_response_rst_stream_error_code(uint32_t error_code)
 {
   response_rst_stream_error_code_ = error_code;
 }
diff --git a/src/shrpx_downstream.h b/src/shrpx_downstream.h
index 76244644..406b6b7f 100644
--- a/src/shrpx_downstream.h
+++ b/src/shrpx_downstream.h
@@ -215,8 +215,8 @@ public:
   evbuffer* get_response_body_buf();
   void add_response_bodylen(size_t amount);
   int64_t get_response_bodylen() const;
-  int get_response_rst_stream_error_code() const;
-  void set_response_rst_stream_error_code(int error_code);
+  uint32_t get_response_rst_stream_error_code() const;
+  void set_response_rst_stream_error_code(uint32_t error_code);
   // Inspects HTTP/1 response.  This checks tranfer-encoding etc.
   void inspect_http1_response();
   // Clears some of member variables for response.
@@ -311,7 +311,7 @@ private:
   int32_t downstream_stream_id_;
 
   // RST_STREAM error_code from downstream HTTP2 connection
-  int response_rst_stream_error_code_;
+  uint32_t response_rst_stream_error_code_;
 
   int request_state_;
   int request_major_;
diff --git a/src/shrpx_http2_downstream_connection.cc b/src/shrpx_http2_downstream_connection.cc
index 3afaaf87..24471025 100644
--- a/src/shrpx_http2_downstream_connection.cc
+++ b/src/shrpx_http2_downstream_connection.cc
@@ -153,7 +153,7 @@ void Http2DownstreamConnection::detach_downstream(Downstream *downstream)
 }
 
 int Http2DownstreamConnection::submit_rst_stream(Downstream *downstream,
-                                                 nghttp2_error_code error_code)
+                                                 uint32_t error_code)
 {
   int rv = -1;
   if(http2session_->get_state() == Http2Session::CONNECTED &&
diff --git a/src/shrpx_http2_downstream_connection.h b/src/shrpx_http2_downstream_connection.h
index 494e337b..afa727d6 100644
--- a/src/shrpx_http2_downstream_connection.h
+++ b/src/shrpx_http2_downstream_connection.h
@@ -71,8 +71,7 @@ public:
   StreamData* detach_stream_data();
 
   int submit_rst_stream
-  (Downstream *downstream,
-   nghttp2_error_code error_code = NGHTTP2_INTERNAL_ERROR);
+  (Downstream *downstream, uint32_t error_code = NGHTTP2_INTERNAL_ERROR);
 private:
   Http2Session *http2session_;
   evbuffer *request_body_buf_;
diff --git a/src/shrpx_http2_session.cc b/src/shrpx_http2_session.cc
index ff5034d1..d17bf5a3 100644
--- a/src/shrpx_http2_session.cc
+++ b/src/shrpx_http2_session.cc
@@ -669,8 +669,7 @@ int Http2Session::submit_request(Http2DownstreamConnection *dconn,
   return 0;
 }
 
-int Http2Session::submit_rst_stream(int32_t stream_id,
-                                   nghttp2_error_code error_code)
+int Http2Session::submit_rst_stream(int32_t stream_id, uint32_t error_code)
 {
   assert(state_ == CONNECTED);
   if(LOG_ENABLED(INFO)) {
@@ -755,7 +754,7 @@ void call_downstream_readcb(Http2Session *http2session, Downstream *downstream)
 
 namespace {
 int on_stream_close_callback
-(nghttp2_session *session, int32_t stream_id, nghttp2_error_code error_code,
+(nghttp2_session *session, int32_t stream_id, uint32_t error_code,
  void *user_data)
 {
   auto http2session = static_cast<Http2Session*>(user_data);
@@ -1632,7 +1631,7 @@ void Http2Session::set_state(int state)
   state_ = state;
 }
 
-int Http2Session::terminate_session(nghttp2_error_code error_code)
+int Http2Session::terminate_session(uint32_t error_code)
 {
   int rv;
   rv = nghttp2_session_terminate_session(session_, error_code);
diff --git a/src/shrpx_http2_session.h b/src/shrpx_http2_session.h
index be40c9bc..226585f6 100644
--- a/src/shrpx_http2_session.h
+++ b/src/shrpx_http2_session.h
@@ -68,11 +68,11 @@ public:
                      int32_t pri, const nghttp2_nv *nva, size_t nvlen,
                      const nghttp2_data_provider *data_prd);
 
-  int submit_rst_stream(int32_t stream_id, nghttp2_error_code error_code);
+  int submit_rst_stream(int32_t stream_id, uint32_t error_code);
 
   int submit_priority(Http2DownstreamConnection *dconn, int32_t pri);
 
-  int terminate_session(nghttp2_error_code error_code);
+  int terminate_session(uint32_t error_code);
 
   nghttp2_session* get_session() const;
 
diff --git a/src/shrpx_http2_upstream.cc b/src/shrpx_http2_upstream.cc
index 1af40c8c..7649720d 100644
--- a/src/shrpx_http2_upstream.cc
+++ b/src/shrpx_http2_upstream.cc
@@ -52,7 +52,7 @@ const size_t INBUF_MAX_THRES = 16*1024;
 
 namespace {
 int on_stream_close_callback
-(nghttp2_session *session, int32_t stream_id, nghttp2_error_code error_code,
+(nghttp2_session *session, int32_t stream_id, uint32_t error_code,
  void *user_data)
 {
   auto upstream = static_cast<Http2Upstream*>(user_data);
@@ -622,15 +622,14 @@ int on_unknown_frame_recv_callback(nghttp2_session *session,
 } // namespace
 
 namespace {
-nghttp2_error_code infer_upstream_rst_stream_error_code
-(int downstream_error_code)
+uint32_t infer_upstream_rst_stream_error_code(uint32_t downstream_error_code)
 {
   // NGHTTP2_REFUSED_STREAM is important because it tells upstream
   // client to retry.
   switch(downstream_error_code) {
   case NGHTTP2_NO_ERROR:
   case NGHTTP2_REFUSED_STREAM:
-    return static_cast<nghttp2_error_code>(downstream_error_code);
+    return downstream_error_code;
   default:
     return NGHTTP2_INTERNAL_ERROR;
   }
@@ -1045,8 +1044,7 @@ void downstream_eventcb(bufferevent *bev, short events, void *ptr)
 }
 } // namespace
 
-int Http2Upstream::rst_stream(Downstream *downstream,
-                              nghttp2_error_code error_code)
+int Http2Upstream::rst_stream(Downstream *downstream, uint32_t error_code)
 {
   if(LOG_ENABLED(INFO)) {
     ULOG(INFO, this) << "RST_STREAM stream_id="
@@ -1065,7 +1063,7 @@ int Http2Upstream::rst_stream(Downstream *downstream,
   return 0;
 }
 
-int Http2Upstream::terminate_session(nghttp2_error_code error_code)
+int Http2Upstream::terminate_session(uint32_t error_code)
 {
   int rv;
   rv = nghttp2_session_terminate_session(session_, error_code);
diff --git a/src/shrpx_http2_upstream.h b/src/shrpx_http2_upstream.h
index 00b333f6..2c7739d6 100644
--- a/src/shrpx_http2_upstream.h
+++ b/src/shrpx_http2_upstream.h
@@ -60,8 +60,8 @@ public:
 
   nghttp2_session* get_http2_session();
 
-  int rst_stream(Downstream *downstream, nghttp2_error_code error_code);
-  int terminate_session(nghttp2_error_code error_code);
+  int rst_stream(Downstream *downstream, uint32_t error_code);
+  int terminate_session(uint32_t error_code);
   int error_reply(Downstream *downstream, unsigned int status_code);
 
   virtual void pause_read(IOCtrlReason reason);
diff --git a/src/shrpx_spdy_upstream.cc b/src/shrpx_spdy_upstream.cc
index 8b63a71e..4c0d9b79 100644
--- a/src/shrpx_spdy_upstream.cc
+++ b/src/shrpx_spdy_upstream.cc
@@ -420,7 +420,7 @@ void on_unknown_ctrl_recv_callback(spdylay_session *session,
 namespace {
 // Infer upstream RST_STREAM status code from downstream HTTP/2
 // error code.
-uint32_t infer_upstream_rst_stream_status_code(int downstream_error_code)
+uint32_t infer_upstream_rst_stream_status_code(uint32_t downstream_error_code)
 {
   // Only propagate *_REFUSED_STREAM so that upstream client can
   // resend request.
diff --git a/tests/nghttp2_frame_test.c b/tests/nghttp2_frame_test.c
index beb7baa7..21f6c710 100644
--- a/tests/nghttp2_frame_test.c
+++ b/tests/nghttp2_frame_test.c
@@ -260,7 +260,7 @@ void test_nghttp2_frame_pack_rst_stream(void)
   nghttp2_frame_rst_stream_free(&oframe);
   nghttp2_bufs_reset(&bufs);
 
-  /* Unknown error code is treated as NGHTTP2_INTERNAL_ERROR */
+  /* Unknown error code is passed to callback as is */
   frame.error_code = 1000000009;
   rv = nghttp2_frame_pack_rst_stream(&bufs, &frame);
 
@@ -270,7 +270,7 @@ void test_nghttp2_frame_pack_rst_stream(void)
   check_frame_header(4, NGHTTP2_RST_STREAM, NGHTTP2_FLAG_NONE, 1000000007,
                      &oframe.hd);
 
-  CU_ASSERT(NGHTTP2_INTERNAL_ERROR == oframe.error_code);
+  CU_ASSERT(1000000009 == oframe.error_code);
 
   nghttp2_frame_rst_stream_free(&oframe);
 
@@ -424,7 +424,7 @@ void test_nghttp2_frame_pack_goaway()
   nghttp2_frame_goaway_free(&oframe);
   nghttp2_bufs_reset(&bufs);
 
-  /* Unknown error code is treated as NGHTTP2_INTERNAL_ERROR */
+  /* Unknown error code is passed to callback as is */
   frame.error_code = 1000000009;
 
   rv = nghttp2_frame_pack_goaway(&bufs, &frame);
@@ -432,7 +432,7 @@ void test_nghttp2_frame_pack_goaway()
   CU_ASSERT(0 == rv);
   CU_ASSERT(0 == unpack_framebuf((nghttp2_frame*)&oframe, &bufs));
   check_frame_header(24, NGHTTP2_GOAWAY, NGHTTP2_FLAG_NONE, 0, &oframe.hd);
-  CU_ASSERT(NGHTTP2_INTERNAL_ERROR == oframe.error_code);
+  CU_ASSERT(1000000009 == oframe.error_code);
 
   nghttp2_frame_goaway_free(&oframe);