Trigger function failure

I have a trigger function problem that threatens to cause my early demise.
Please someone help an absolute novice.

I have two tables, invoice and invline, with the usual functions.

Table invoice has a column for the total of debit lines, and a column for
the total of credit (payment) lines. The function is triggered by any
change in inv.gross

The ERROR occurs if there are no invline found by either summing part,
where we set dr_total or cr_total.
If I change "sum" to "count" it works as expected, returning 0.00

--THE FUNCTION
UPDATE invoice SET
invoice.cr_total = (
SELECT
sum(invline.gross)
FROM invline
WHERE invline.type > 4 AND invline.invoice_id = 200003
)
,
invoice.dr_total = (
SELECT
sum(invline.gross)
FROM invline
WHERE invline.type <5 AND invline.invoice_id = 200003
)
WHERE invoice.id = 200003;

--TEST
select invoice.dr_total, invoice.cr_total from invoice where invoice.id
=200003

---------------------------------------------------------------------------------------------

ERROR: null value in column "po_cr_total" violates not-null constraint

Anyone?

Mike