diff --git a/lib/nghttp2_session.c b/lib/nghttp2_session.c
index 7eb6b642..048b0a2a 100644
--- a/lib/nghttp2_session.c
+++ b/lib/nghttp2_session.c
@@ -1597,11 +1597,16 @@ int nghttp2_session_send(nghttp2_session *session)
         return NGHTTP2_ERR_CALLBACK_FAILURE;
       }
     } else {
-      session->aob.framebufoff += sentlen;
-      if(session->aob.item->frame_cat == NGHTTP2_CAT_DATA) {
+      if(session->aob.item->frame_cat == NGHTTP2_CAT_DATA &&
+         session->aob.framebufoff + sentlen > NGHTTP2_FRAME_HEAD_LENGTH) {
         nghttp2_data *frame;
         nghttp2_stream *stream;
-        uint16_t len = nghttp2_get_uint16(&session->aob.framebuf[0]);
+        uint16_t len;
+        if(session->aob.framebufoff < NGHTTP2_FRAME_HEAD_LENGTH) {
+          len = session->aob.framebufoff + sentlen - NGHTTP2_FRAME_HEAD_LENGTH;
+        } else {
+          len = sentlen;
+        }
         frame = nghttp2_outbound_item_get_data_frame(session->aob.item);
         stream = nghttp2_session_get_stream(session, frame->hd.stream_id);
         if(stream && stream->remote_flow_control) {
@@ -1611,6 +1616,7 @@ int nghttp2_session_send(nghttp2_session *session)
           session->remote_window_size -= len;
         }
       }
+      session->aob.framebufoff += sentlen;
       if(session->aob.framebufoff == session->aob.framebuflen) {
         /* Frame has completely sent */
         r = nghttp2_session_after_frame_sent(session);
diff --git a/tests/nghttp2_session_test.c b/tests/nghttp2_session_test.c
index 8dd3be2f..71699112 100644
--- a/tests/nghttp2_session_test.c
+++ b/tests/nghttp2_session_test.c
@@ -68,6 +68,7 @@ typedef struct {
   int data_chunk_recv_cb_called;
   int data_recv_cb_called;
   const nghttp2_frame *frame;
+  size_t fixed_sendlen;
 } my_user_data;
 
 static void scripted_data_feed_init(scripted_data_feed *df,
@@ -94,6 +95,15 @@ static ssize_t fail_send_callback(nghttp2_session *session,
   return NGHTTP2_ERR_CALLBACK_FAILURE;
 }
 
+static ssize_t fixed_bytes_send_callback(nghttp2_session *session,
+                                         const uint8_t *data, size_t len,
+                                         int flags, void *user_data)
+{
+  size_t fixed_sendlen = ((my_user_data*)user_data)->fixed_sendlen;
+  return fixed_sendlen < len ? fixed_sendlen : len;
+}
+
+
 static ssize_t scripted_recv_callback(nghttp2_session *session,
                                       uint8_t* data, size_t len, int flags,
                                       void *user_data)
@@ -2949,12 +2959,15 @@ void test_nghttp2_session_flow_control(void)
   nghttp2_frame settings_frame;
 
   memset(&callbacks, 0, sizeof(nghttp2_session_callbacks));
-  callbacks.send_callback = null_send_callback;
+  callbacks.send_callback = fixed_bytes_send_callback;
   callbacks.on_frame_send_callback = on_frame_send_callback;
   data_prd.read_callback = fixed_length_data_source_read_callback;
 
   ud.frame_send_cb_called = 0;
   ud.data_source_length = 128*1024;
+  /* Use smaller emission count so that we can check outbound flow
+     control window calculation is correct. */
+  ud.fixed_sendlen = 2*1024;
 
   /* Initial window size to 64KiB - 1*/
   nghttp2_session_client_new(&session, &callbacks, &ud);