diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index d2a3fcd461c..aa62641bc26 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -8046,7 +8046,7 @@ function message_popup_window() {
     $popuplimit = 30;     // Minimum seconds between popups
 
     if (!defined('MESSAGE_WINDOW')) {
-        if (isset($USER->id) and !isguestuser()) {
+        if (!empty($USER->id) and !isguestuser()) {
             if (!isset($USER->message_lastpopup)) {
                 $USER->message_lastpopup = 0;
             }