[thelist] SQL: checking for an empty date field

I am trying to execute the following SQL query:

SELECT * FROM Invoices WHERE Paid = Null AND SellerType = 1

"Paid" is a Date/Time field in an Access db.  On Invoices that haven't been
paid, the field is blank (nothing has ever been entered into the field).

When I enter this statement directly into Access as a query, it works fine,
and returns exactly the records I want.

When I try to run it from an ASP page, I don't get errors, but it returns no
records, which is not correct.

How should I be doing this?