diff --git a/_sources/apiref.txt b/_sources/apiref.txt
index 51269e2b..3fb0838a 100644
--- a/_sources/apiref.txt
+++ b/_sources/apiref.txt
@@ -722,7 +722,8 @@ Types (structs, unions and typedefs)
without blocking, it must return :macro:`NGHTTP2_ERR_WOULDBLOCK`. If
it gets EOF before it reads any single byte, it must return
:macro:`NGHTTP2_ERR_EOF`. For other errors, it must return
- :macro:`NGHTTP2_ERR_CALLBACK_FAILURE`.
+ :macro:`NGHTTP2_ERR_CALLBACK_FAILURE`. Returning 0 is treated as
+ :macro:`NGHTTP2_ERR_WOULDBLOCK`.
.. type:: typedef int (*nghttp2_on_frame_recv_callback) (nghttp2_session *session, const nghttp2_frame *frame, void *user_data)
diff --git a/apiref.html b/apiref.html
index a68da549..126fd4f7 100644
--- a/apiref.html
+++ b/apiref.html
@@ -1113,7 +1113,8 @@ written in buf if it succeeds. If it cannot read any single byte
without blocking, it must return NGHTTP2_ERR_WOULDBLOCK. If
it gets EOF before it reads any single byte, it must return
NGHTTP2_ERR_EOF. For other errors, it must return
-NGHTTP2_ERR_CALLBACK_FAILURE.
+NGHTTP2_ERR_CALLBACK_FAILURE. Returning 0 is treated as
+NGHTTP2_ERR_WOULDBLOCK.
diff --git a/nghttp2.h.html b/nghttp2.h.html
index 0cb5c18f..b792f093 100644
--- a/nghttp2.h.html
+++ b/nghttp2.h.html
@@ -860,7 +860,8 @@
* without blocking, it must return :enum:`NGHTTP2_ERR_WOULDBLOCK`. If
* it gets EOF before it reads any single byte, it must return
* :enum:`NGHTTP2_ERR_EOF`. For other errors, it must return
- * :enum:`NGHTTP2_ERR_CALLBACK_FAILURE`.
+ * :enum:`NGHTTP2_ERR_CALLBACK_FAILURE`. Returning 0 is treated as
+ * :enum:`NGHTTP2_ERR_WOULDBLOCK`.
*/
typedef ssize_t (*nghttp2_recv_callback)
(nghttp2_session *session,