MVC :: Nullable DateTime Give Invalid Modelstate If Null?

Got a nullable datetime member on my viewmodel.the moment it hit my controller action, the viewmodel's modelstate allready are invalid (because the member is null).Forgot...how that works? (using MVC2)Basically my viewmodel have a social security number, passport number and passport expiracy date.If the ssn is entered, I want to ignore the two passport related fields, so I made the expiracy date member nullable.

Like all other viewmodels in this app, each model have it's own validation function that will do validation, and in there I add an error that the date is required or not valid (if using passport number). mvc itself should not care about the date at all.I dont want to give the date a default value, because then the date will show when the form loads, and it must be an empty field (unless I already got a date of course.