diff --git a/lib/includes/nghttp2/nghttp2.h b/lib/includes/nghttp2/nghttp2.h
index 926d730d..09e2dba5 100644
--- a/lib/includes/nghttp2/nghttp2.h
+++ b/lib/includes/nghttp2/nghttp2.h
@@ -1747,11 +1747,9 @@ typedef int (*nghttp2_unpack_extension_callback)(nghttp2_session *session,
  * extension payload in its wire format.  The frame header will be
  * packed by library.  Application must pack payload only.
  * frame->ext.payload is the object passed to
- * `nghttp2_submit_extension()` as payload parameter.  The |*flags| is
- * initialized to the flags parameter passed in
- * `nghttp2_submit_extension()`.  Application can modify flags value
- * if it wants.  Application must pack extension payload to the |buf|
- * of its capacity |len| bytes.
+ * `nghttp2_submit_extension()` as payload parameter.  Application
+ * must pack extension payload to the |buf| of its capacity |len|
+ * bytes.
  *
  * The implementation of this function returns the number of bytes
  * written into |buf| when it succeeds.
@@ -1770,8 +1768,7 @@ typedef int (*nghttp2_unpack_extension_callback)(nghttp2_session *session,
  * :enum:`NGHTTP2_ERR_CALLBACK_FAILURE`.
  */
 typedef ssize_t (*nghttp2_pack_extension_callback)(nghttp2_session *session,
-                                                   uint8_t *flags, uint8_t *buf,
-                                                   size_t len,
+                                                   uint8_t *buf, size_t len,
                                                    const nghttp2_frame *frame,
                                                    void *user_data);
 
diff --git a/lib/nghttp2_session.c b/lib/nghttp2_session.c
index 9c6d34ac..84f90a5b 100644
--- a/lib/nghttp2_session.c
+++ b/lib/nghttp2_session.c
@@ -1738,19 +1738,17 @@ static size_t session_estimate_headers_payload(nghttp2_session *session,
 static int session_pack_extension(nghttp2_session *session, nghttp2_bufs *bufs,
                                   nghttp2_frame *frame) {
   ssize_t rv;
-  uint8_t flags;
   nghttp2_buf *buf;
   size_t buflen;
   size_t framelen;
 
   assert(session->callbacks.pack_extension_callback);
 
-  flags = frame->hd.flags;
   buf = &bufs->head->buf;
   buflen = nghttp2_min(nghttp2_buf_avail(buf), NGHTTP2_MAX_PAYLOADLEN);
 
-  rv = session->callbacks.pack_extension_callback(
-      session, &flags, buf->last, buflen, frame, session->user_data);
+  rv = session->callbacks.pack_extension_callback(session, buf->last, buflen,
+                                                  frame, session->user_data);
   if (rv == NGHTTP2_ERR_CANCEL) {
     return (int)rv;
   }
@@ -1761,7 +1759,6 @@ static int session_pack_extension(nghttp2_session *session, nghttp2_bufs *bufs,
 
   framelen = (size_t)rv;
 
-  frame->hd.flags = flags;
   frame->hd.length = framelen;
 
   assert(buf->pos == buf->last);
diff --git a/tests/nghttp2_session_test.c b/tests/nghttp2_session_test.c
index dcde6d93..86c9e711 100644
--- a/tests/nghttp2_session_test.c
+++ b/tests/nghttp2_session_test.c
@@ -435,8 +435,7 @@ static int on_stream_close_callback(nghttp2_session *session _U_,
 }
 
 static ssize_t pack_extension_callback(nghttp2_session *session _U_,
-                                       uint8_t *flags _U_, uint8_t *buf,
-                                       size_t len _U_,
+                                       uint8_t *buf, size_t len _U_,
                                        const nghttp2_frame *frame,
                                        void *user_data _U_) {
   nghttp2_buf *p = frame->ext.payload;