MADateDescription and nil objects

I have a couple of MADateDescription to manage the effective date and the expiration date of a structure and their values could be either a valid date or nil. The problem is that once a date object is assigned to these variables through the fields generated by MADateDescription, it is not possible to assign a nil value just by deleting the content of the fields. MADateDescription always tries to parse the field as a date. 1. Left the field empty, effectiveDate is nil. 2. Input a date in the field, effectiveDate is a valid date. 3. Delete the content of the field, MADateDescription complains about an invalid date format. Is there a way to avoid that parsing?