diff --git a/lib/includes/nghttp2/nghttp2.h b/lib/includes/nghttp2/nghttp2.h
index 14f8950b..e5217fec 100644
--- a/lib/includes/nghttp2/nghttp2.h
+++ b/lib/includes/nghttp2/nghttp2.h
@@ -5337,7 +5337,7 @@ nghttp2_stream_get_sum_dependency_weight(nghttp2_stream *stream);
  * ``DEBUGBUILD`` macro defined.
  */
 typedef void (*nghttp2_debug_vprintf_callback)(const char *format,
-                                               va_list args);
+              va_list args)  __attribute__ ((format(printf, 1, 0)));
 
 /**
  * @function