diff --git a/comment/comment_ajax.php b/comment/comment_ajax.php
index 131a803888e78..c1fe37c00bd82 100644
--- a/comment/comment_ajax.php
+++ b/comment/comment_ajax.php
@@ -22,6 +22,7 @@
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 define('AJAX_SCRIPT', true);
+define('NO_DEBUG_DISPLAY', true);
 
 require_once('../config.php');
 require_once($CFG->dirroot . '/comment/lib.php');