Please Help with TBDEDataSet.Locate

This has probably been previously answered but I didn't find it in the
headers.  Can anyone please help with how to find a blank value in the
locate procedure?  Example codes follows:

    carda and cardb are known values and I want the return date field to be
blank (no value).  Cardno_a and Cardno_b are Paradox short integer fields,
Return_Date field is a Paradox Date field.

                if locate('Cardno_a; Cardno_b;Return_Date',
VarArrayOf([carda,cardb, ?]),([])) = False then
                       {if this card isn't present then message displayed}
                     Application.messagebox('Card Number Not Found', 'Card
Search', 1);

Your Assistance is appreciated.