Passing a null value in pl/pgsql

I have two stored procedures and one calls the other.  If
the caller passes a null for one of the parameters (the fourth) a delete
query in the called function will not work.

If I pass a null as the 4th parameter (_seqSalesOrderLine) the delete
query below will not match any records, and my business rule fails.  If I
temporarily (for testing only) change the asterisked line to read
   and seqSalesOrderLine = null
the query works as I would expect.

Am I missing something (please tell me) or is this a bug when passing in a
null value?

Thanks for any help,
Mike.


Bits of called function (asterisks mark important lines)
===========================================================================
    create function salesOrderTax_apply(int8,int8,
                                        text,int8,float8,
                                        int8,text) returns integer as '
        declare
            ...
*           _seqSalesOrderLine  alias for $4;
            ...
        begin
            ...
            delete from salesOrderTax
            where seqSalesOrder = soRec.seqSalesOrder
            and codeSalesOrderTax = _codeSalesOrderTax
*           and seqSalesOrderLine = _seqSalesOrderLine
            ;
            ...
            return null; --bogus return value
        end;
    ' language 'plpgsql';


Bits of calling function (asterisks mark important lines)
===========================================================================
    create function salesOrderControl_freightTax(int8, int8,text)
                                                  returns integer as '
        declare
            ...
            _seqTaxSchedule    int8;
            ...
        begin
            ...
            -- get the appropriate tax schedule
            select seqTaxSchedule
            into _seqTaxSchedule
            from carrier
            where name = soRec.carrier_name
            ;
            ...
            perform
               salesOrderTax_apply(soRec.seqSalesOrder, _seqTaxSchedule,
*                                  ''Freight'', null, socRec.totalFreight,
                                   _txSeq, _txUser
                                  );

            return null; --bogus return value
        end;
    ' language 'plpgsql';


=========================================
Mike Finn
mike.finn@tacticalExecutive.com