problem with datetimepiker

Dear every one Regards

am new in vb.net 

my problem is this!

i developed a windows form if i click on datetimepiker and then select a date then ok

if i do not click any date then massage show

Additional information: The conversion of a varchar data type to a datetime data type resulted in an out-of-range value.

i have field name with opdate and its data type is also datetime

please help me

thank