diff --git a/mod/calendar/class/Admin.php b/mod/calendar/class/Admin.php
index f7102c713..42ca48615 100644
--- a/mod/calendar/class/Admin.php
+++ b/mod/calendar/class/Admin.php
@@ -169,6 +169,7 @@ public static function event_form(Calendar_Event $event, $suggest = false)
         $form->addText('summary', $event->summary);
         $form->setLabel('summary', 'Summary');
         $form->setSize('summary', 60);
+        $form->setRequired('summary');
 
         $form->addText('location', $event->location);
         $form->setLabel('location', 'Location');