diff --git a/classes/page_with_filter_controls.php b/classes/page_with_filter_controls.php
index 22f7ab5..6e1fe91 100644
--- a/classes/page_with_filter_controls.php
+++ b/classes/page_with_filter_controls.php
@@ -171,7 +171,7 @@ class mod_attendance_page_with_filter_controls {
     private function calc_sessgroupslist_sesstype() {
         global $SESSION;
 
-        if (!array_key_exists('attsessiontype', $SESSION)) {
+        if (!property_exists('attsessiontype', $SESSION)) {
             $SESSION->attsessiontype = array($this->cm->course => self::SESSTYPE_ALL);
         } else if (!array_key_exists($this->cm->course, $SESSION->attsessiontype)) {
             $SESSION->attsessiontype[$this->cm->course] = self::SESSTYPE_ALL;