diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index 4a8b0e83..0e61615f 100644
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -1278,7 +1278,7 @@ class calendar extends rcube_plugin
         case 'EXDATE':
           foreach ((array)$val as $i => $ex)
             $val[$i] = gmdate('Ymd\THis', $ex);
-          $val = join(',', $val);
+          $val = join(',', (array)$val);
           break;
       }
       $rrule .= $k . '=' . $val . ';';
diff --git a/plugins/calendar/calendar_ui.js b/plugins/calendar/calendar_ui.js
index 012d176c..75333a01 100644
--- a/plugins/calendar/calendar_ui.js
+++ b/plugins/calendar/calendar_ui.js
@@ -2437,7 +2437,7 @@ function rcube_calendar_ui(settings)
       /* Time completions */
       var result = [];
       var now = new Date();
-      var st, start = (this.element.attr('id').indexOf('endtime') > 0
+      var st, start = (String(this.element.attr('id')).indexOf('endtime') > 0
         && (st = $('#edit-starttime').val())
         && $('#edit-startdate').val() == $('#edit-enddate').val())
         ? parse_datetime(st, '') : null;