diff --git a/src/directives/mwlCalendar.js b/src/directives/mwlCalendar.js
index 73229b0c..f9588dbb 100644
--- a/src/directives/mwlCalendar.js
+++ b/src/directives/mwlCalendar.js
@@ -118,9 +118,9 @@ angular
         view: '=',
         viewTitle: '=?',
         viewDate: '=',
-        editEventHtml: '=',
-        deleteEventHtml: '=',
-        cellIsOpen: '=',
+        editEventHtml: '=?',
+        deleteEventHtml: '=?',
+        cellIsOpen: '=?',
         onEventClick: '&',
         onEventTimesChanged: '&',
         onEditEventClick: '&',