diff --git a/mod/chat/gui_ajax/update.php b/mod/chat/gui_ajax/update.php
index f2780d738c8..3ad91792e93 100755
--- a/mod/chat/gui_ajax/update.php
+++ b/mod/chat/gui_ajax/update.php
@@ -5,6 +5,8 @@
  * @license http://www.gnu.org/copyleft/gpl.html GNU Public License
  */
 
+define('NO_MOODLE_COOKIES', true); // session not used here
+
 require_once('../../../config.php');
 require_once('../lib.php');
 require_once('common.php');