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
@@ -596,7 +596,11 @@
             $event = $this->get_event_from_invitation($request);
 
             // find the event in calendar
-            $existing = $this->find_event_by_uid($event['uid']);
+            if (array_key_exists('uid', $event)) {
+                $existing = $this->find_event_by_uid($event['uid']);
+            } else {
+                $existing = $this->find_event_by_uid($event->uID);
+            }
 /*
             switch ($status) {
                 case 'ACCEPTED':  $event['free_busy'] = 'busy';      break;
@@ -665,7 +669,11 @@
         //        as it's expected by the specification. Server
         //        should delete an event in such a case, but we
         //        keep the event copy with appropriate attendee status instead.
-        return empty($status) ? null : $this->serverId($event['uid'], $folder);
+        return empty($status) ? null : (
+            array_key_exists('uid', $event) ?
+                $this->serverId($event['uid'], $folder) :
+                $this->serverId($event->uID, $folder)
+        );
     }
 
     /**
@@ -684,6 +692,14 @@
                 return $event;
             }
 
+            $collection = new Syncroton_Model_SyncCollection(
+                array('collectionId' => $request->collectionId)
+            );
+
+            if ($event = $this->getEntry($collection, $request->requestId)) {
+                return $event;
+            }
+
             throw new Syncroton_Exception_Status_MeetingResponse(Syncroton_Exception_Status_MeetingResponse::INVALID_REQUEST);
         }
 
@@ -723,8 +739,10 @@
 
         $this->update_attendee_status($old, $status);
 
-        if ($event['free_busy']) {
-            $old['free_busy'] = $event['free_busy'];
+        if (array_key_exists('free_busy', $event)) {
+            if ($event['free_busy']) {
+                $old['free_busy'] = $event['free_busy'];
+            }
         }
 
         // Updating an existing event is most-likely a response