diff --git a/lib/includes/spdylay/spdylay.h b/lib/includes/spdylay/spdylay.h
index 7ddc8aee..f9029e2f 100644
--- a/lib/includes/spdylay/spdylay.h
+++ b/lib/includes/spdylay/spdylay.h
@@ -538,7 +538,7 @@ int spdylay_submit_response(spdylay_session *session,
  *     Out of memory.
  */
 int spdylay_submit_syn_stream(spdylay_session *session, uint8_t flags,
-                              uint32_t assoc_stream_id, uint8_t pri,
+                              int32_t assoc_stream_id, uint8_t pri,
                               const char **nv, void *stream_user_data);
 
 /*
diff --git a/lib/spdylay_submit.c b/lib/spdylay_submit.c
index 920bf3fc..a718ffaf 100644
--- a/lib/spdylay_submit.c
+++ b/lib/spdylay_submit.c
@@ -99,7 +99,7 @@ static int spdylay_submit_syn_stream_shared
 }
 
 int spdylay_submit_syn_stream(spdylay_session *session, uint8_t flags,
-                              uint32_t assoc_stream_id, uint8_t pri,
+                              int32_t assoc_stream_id, uint8_t pri,
                               const char **nv, void *stream_user_data)
 {
   return spdylay_submit_syn_stream_shared(session, flags, assoc_stream_id,