diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index f33267a8..94e1e41d 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -1351,7 +1351,7 @@ class kolab_driver extends calendar_driver
     $record['id'] = $record['uid'];
 
     // all-day events go from 12:00 - 13:00
-    if ($record['end'] <= $record['start'] && $record['allday']) {
+    if (is_a($record['start'], 'DateTime') && $record['end'] <= $record['start'] && $record['allday']) {
       $record['end'] = clone $record['start'];
       $record['end']->add(new DateInterval('PT1H'));
     }