Query with Date field = Null

I have the following query, which works unless the ExpirationDate filed contains "Null". How would I make it work with Null Values? I'd like it to work where the ExpirationDate is <= or Null. SELECT tblDataPermit.ApplicationNumber ,tblDataPermit.PermitNumber ,tblDataPermit.ExpirationDate ,tblDataPermit.Type ,tblDataPermit.Status ,tblDataPermit.ApplicationType ,tblDataParcel.StreetName ,tblDataParcel.StreetNum ,tblDataParcel.LocationFROM tblDataPermit INNER JOIN tblDataParcel ON tblDataPermit.ParcelID = tblDataParcel.ParcelIDWHERE tblDataPermit.Type = @Type AND tblDataPermit.Status = @Status AND tblDataPermit.ExpirationDate <= @ExpirationDate