diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index caead9b3..dd33a0ba 100644
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -1157,6 +1157,7 @@ class calendar extends rcube_plugin
     }
 
     return array(
+      '_id'   => $event['calendar'] . ':' . $event['id'],  // unique identifier for fullcalendar
       'start' => $this->adjust_timezone($event['start'])->format('c'),
       'end'   => $this->adjust_timezone($event['end'])->format('c'),
       'title'       => strval($event['title']),
diff --git a/plugins/calendar/calendar_ui.js b/plugins/calendar/calendar_ui.js
index 07c3a0c7..4a54ca54 100644
--- a/plugins/calendar/calendar_ui.js
+++ b/plugins/calendar/calendar_ui.js
@@ -2006,7 +2006,7 @@ function rcube_calendar_ui(settings)
         var event = p.update;
         event.temp = false;
         event.editable = source.editable;
-        var existing = fc.fullCalendar('clientEvents', function(e){ return (e.id == event.id && e.calendar == event.calendar); });
+        var existing = fc.fullCalendar('clientEvents', event._id);
         if (existing.length) {
           $.extend(existing[0], event);
           fc.fullCalendar('updateEvent', existing[0]);