Testing for a Null date/Time field

This is driving me mad, it should be all so simple, I have a date/time field on an Access form that I want to test if it is unfilled (null), if so put a check in a checkbox field, so ..

if me.reminded = null then

is what I'm doing, but it seems that the field is NEVER null (even though it is, str(me.reminded) gives an "Invalid use of Null" error)

Someone help before this laptop goes through the window