diff --git a/plugins/calendar/lib/calendar_itip.php b/plugins/calendar/lib/calendar_itip.php
index 6ff5b6b..6b21492 100644
--- a/plugins/calendar/lib/calendar_itip.php
+++ b/plugins/calendar/lib/calendar_itip.php
@@ -159,7 +159,9 @@ class calendar_itip
     // attach ics file for this event
     $ical = $this->cal->get_ical();
     $ics = $ical->export(array($event), $method);
-    $message->addAttachment($ics, 'text/calendar', 'event.ics', false, '8bit', '', RCMAIL_CHARSET . "; method=" . $method);
+    $message->addAttachment($ics, 'text/calendar', 'event.ics', false, '8bit', '', RCMAIL_CHARSET . "; method=" . $method,
+        '', '', null, null, '', null,  // language, location, n_encoding, f_encoding, description, h_charset
+        array('Content-class' => 'urn:content-classes:calendarmessage'));
     
     return $message;
   }