fullcalendar JavaScript runtime error: Unable to get property 'format' of undefined or null reference

I am trying to format a date in fullcalendar. On create I am getting an error 0x800a138f - JavaScript runtime error: Unable to get property 'format' of undefined or null reference. But on Edit it works.

$('#txtStart').val(selectedEvent.start.format('DD-MMM-YYYY'));