diff --git a/plugins/calendar/drivers/kolab/kolab_invitation_calendar.php b/plugins/calendar/drivers/kolab/kolab_invitation_calendar.php
index b48e7265..4a440747 100644
--- a/plugins/calendar/drivers/kolab/kolab_invitation_calendar.php
+++ b/plugins/calendar/drivers/kolab/kolab_invitation_calendar.php
@@ -257,7 +257,7 @@ class kolab_invitation_calendar
         }
 
         if ($match) {
-          $events[$event['uid']] = $this->_mod_event($event);
+          $events[$event['id'] ?: $event['uid']] = $this->_mod_event($event);
         }
       }