diff --git a/lib/kolab_sync_data_calendar.php b/lib/kolab_sync_data_calendar.php
--- a/lib/kolab_sync_data_calendar.php
+++ b/lib/kolab_sync_data_calendar.php
@@ -691,7 +691,11 @@
         }
 
         // extract event from the invitation
-        [$event, $existing] = $this->get_event_from_invitation($request);
+        try {
+            [$event, $existing] = $this->get_event_from_invitation($request);
+        } catch (Exception $e) {
+            throw new Syncroton_Exception_Status_MeetingResponse(Syncroton_Exception_Status_MeetingResponse::MEETING_ERROR);
+        }
         /*
                     switch ($status) {
                         case 'ACCEPTED':  $event['free_busy'] = 'busy';      break;