diff --git a/plugins/libkolab/lib/kolab_date_recurrence.php b/plugins/libkolab/lib/kolab_date_recurrence.php
index 85ffd912..1dc63bef 100644
--- a/plugins/libkolab/lib/kolab_date_recurrence.php
+++ b/plugins/libkolab/lib/kolab_date_recurrence.php
@@ -118,7 +118,7 @@ class kolab_date_recurrence
         }
 
         // determine a reasonable end date if none given
-        if (!$event['recurrence']['COUNT']) {
+        if (!$event['recurrence']['COUNT'] && $event['start'] instanceof DateTime) {
           switch ($event['recurrence']['FREQ']) {
             case 'YEARLY':  $intvl = 'P100Y'; break;
             case 'MONTHLY': $intvl = 'P20Y';  break;