trigger parameters, what am I doing wrong ??

Hi guys. I'm trying to pass a parameter to trigger procedure but it's not
working and I have no idea why since it looks pretty ok. Here's what I do :

CREATE OR REPLACE FUNCTION test_proc()
RETURNS "trigger" AS
$BODY$
DECLARE
chk boolean;
par integer := TG_ARGV[0];

BEGIN
RAISE NOTICE 'TG_ARGV = %, TG_NARGS = %, par = %', TG_ARGV[0], TG_NARGS,
par;

-- [...] some computations

RETURN NEW;
END;

$BODY$
LANGUAGE 'plpgsql' VOLATILE;

CREATE TRIGGER jks_test_proc_tg
AFTER UPDATE
ON test_table
FOR EACH ROW
EXECUTE PROCEDURE test_proc(42);

And here's what RAISE NOTICE looks like : NOTICE: TG_ARGV = <NULL>,
TG_NARGS = 0, par = <NULL>

What's wrong with it ?? I'm running 8.1.4

regards
mk