diff --git a/message/externallib.php b/message/externallib.php
index 0c9c0559007..65ccda6c316 100644
--- a/message/externallib.php
+++ b/message/externallib.php
@@ -3196,7 +3196,7 @@ class core_message_external extends external_api {
      * @since 3.6
      */
     public static function mark_all_conversation_messages_as_read_returns() {
-        return new external_warnings();
+        return null;
     }
 
     /**