diff --git a/plugins/libcalendaring/libcalendaring.php b/plugins/libcalendaring/libcalendaring.php
index 94ff7f26..698fb689 100644
--- a/plugins/libcalendaring/libcalendaring.php
+++ b/plugins/libcalendaring/libcalendaring.php
@@ -1520,14 +1520,18 @@ class libcalendaring extends rcube_plugin
                 break;
             case 'RDATE':
             case 'EXDATE':
-                foreach ((array)$val as $i => $ex)
-                    $val[$i] = $ex->format('Ymd\THis');
+                foreach ((array)$val as $i => $ex) {
+                    if (is_a($ex, 'DateTime'))
+                        $val[$i] = $ex->format('Ymd\THis');
+                }
                 $val = join(',', (array)$val);
                 break;
             case 'EXCEPTIONS':
                 continue 2;
             }
-            $rrule .= $k . '=' . $val . ';';
+
+            if (strlen($val))
+                $rrule .= $k . '=' . $val . ';';
         }
 
         return rtrim($rrule, ';');
diff --git a/plugins/libcalendaring/libvcalendar.php b/plugins/libcalendaring/libvcalendar.php
index 826e8d84..37f62241 100644
--- a/plugins/libcalendaring/libvcalendar.php
+++ b/plugins/libcalendaring/libvcalendar.php
@@ -969,10 +969,13 @@ class libvcalendar implements Iterator
             $ve->add('SEQUENCE', $event['sequence']);
 
         if ($event['recurrence'] && !$recurrence_id) {
-            if ($exdates = $event['recurrence']['EXDATE']) {
+            $exdates = $rdates = null;
+            if (isset($event['recurrence']['EXDATE'])) {
+                $exdates = $event['recurrence']['EXDATE'];
                 unset($event['recurrence']['EXDATE']);  // don't serialize EXDATEs into RRULE value
             }
-            if ($rdates = $event['recurrence']['RDATE']) {
+            if (isset($event['recurrence']['RDATE'])) {
+                $rdates = $event['recurrence']['RDATE'];
                 unset($event['recurrence']['RDATE']);  // don't serialize RDATEs into RRULE value
             }
 
@@ -981,7 +984,7 @@ class libvcalendar implements Iterator
             }
 
             // add EXDATEs each one per line (for Thunderbird Lightning)
-            if ($exdates) {
+            if (is_array($exdates)) {
                 foreach ($exdates as $ex) {
                     if ($ex instanceof \DateTime) {
                         $exd = clone $event['start'];
@@ -992,7 +995,7 @@ class libvcalendar implements Iterator
                 }
             }
             // add RDATEs
-            if (!empty($rdates)) {
+            if (is_array($rdates) && !empty($rdates)) {
                 $sample = $this->datetime_prop('RDATE', $rdates[0]);
                 $rdprop = new VObject\Property\MultiDateTime('RDATE', null);
                 $rdprop->setDateTimes($rdates, $sample->getDateType());