diff --git a/plugins/calendar/drivers/kolab/kolab_calendar.php b/plugins/calendar/drivers/kolab/kolab_calendar.php
index 98041c8f..590bb559 100644
--- a/plugins/calendar/drivers/kolab/kolab_calendar.php
+++ b/plugins/calendar/drivers/kolab/kolab_calendar.php
@@ -580,10 +580,8 @@ class kolab_calendar
    */
   private function _from_rcube_event($event, $old = array())
   {
-    $object = &$event;
-
     // in kolab_storage attachments are indexed by content-id
-    $object['_attachments'] = array();
+    $event['_attachments'] = array();
     if (is_array($event['attachments'])) {
       foreach ($event['attachments'] as $idx => $attachment) {
         $key = null;
@@ -600,15 +598,15 @@ class kolab_calendar
 
         // flagged for deletion => set to false
         if ($attachment['_deleted']) {
-          $object['_attachments'][$key] = false;
+          $event['_attachments'][$key] = false;
         }
         // replace existing entry
         else if ($key) {
-          $object['_attachments'][$key] = $attachment;
+          $event['_attachments'][$key] = $attachment;
         }
         // append as new attachment
         else {
-          $object['_attachments'][] = $attachment;
+          $event['_attachments'][] = $attachment;
         }
       }
 
@@ -625,6 +623,9 @@ class kolab_calendar
 
     $event['_owner'] = $identity['email'];
 
+    // remove some internal properties which should not be saved
+    unset($event['_savemode'], $event['_fromcalendar'], $event['_identity']);
+
     // copy meta data (starting with _) from old object
     foreach ((array)$old as $key => $val) {
       if (!isset($event[$key]) && $key[0] == '_')
diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index 12d60c28..6b980cd8 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -610,6 +610,9 @@ class kolab_driver extends calendar_driver
         $event['recurrence'] = array();
         $event['thisandfuture'] = $savemode == 'future';
 
+        // remove some internal properties which should not be saved
+        unset($event['_savemode'], $event['_fromcalendar'], $event['_identity']);
+
         // save properties to a recurrence exception instance
         if ($old['recurrence_id']) {
             $i = $old['_instance'] - 1;