Trigger function

Hi

Although a very noviceish person when it comes to PostgreSQL 9.1, I have
successfully used trigger functions very similar to the following, but this
one is not liked for some reason.
My intention is to update an integer column po_sub_invoice_nr based on the
value in another integer column po_invoice_nr.
This is my trigger:
CREATE TRIGGER inv_nr_ud_trig
AFTER UPDATE
ON purchaseorder
FOR EACH ROW
EXECUTE PROCEDURE inv_nr_ud_trig_fn()

CREATE OR REPLACE inv_nr_ud_trig_fn()

RETURNS trigger AS $body$

BEGIN

UPDATE purchaseorder SET po_sub_invoice_nr =
mod(NEW.po_invoice_nr,10000)

WHERE po_id=OLD.po_id;

RETURN NEW;

END;

$body$

LANGUAGE plpgsql VOLATILE

COST 100;

This results in a stack overflow and, in pgAdminIII the following return
repeated many many times:

SQL statement "UPDATE purchaseorder SET po_sub_invoice_nr =
mod(NEW.po_invoice_nr,10000) WHERE po_id=OLD.po_id"

PL/pgSQL function "inv_nr_ud_trig_fn" line 4 at SQL statement

I'd be grateful for any pointers.

Mike