[7.3.x] function does not exist ... ?

'k, this doesn't look right, but it could be that I'm overlooking
something ...

The function I created:

CREATE FUNCTION month_trunc (timestamp without time zone) RETURNS timestamp without time zone
AS 'SELECT date_trunc(''month'', $1 )'
LANGUAGE sql IMMUTABLE;

The query that fails:

ams=# select * from traffic_logs where month_trunc(runtime) = month_trunc(now());
ERROR: Function month_trunc(timestamp with time zone) does not exist
Unable to identify a function that satisfies the given argument types
You may need to add explicit typecasts

The query that succeeds:

ams=# explain select * from traffic_logs where month_trunc(runtime) = '2003-10-01';
QUERY PLAN
-------------------------------------------------------------------------------------------
Index Scan using tl_month on traffic_logs (cost=0.00..30751.90 rows=8211 width=36)
Index Cond: (month_trunc(runtime) = '2003-10-01 00:00:00'::timestamp without time zone)
(2 rows)

I haven't mis-spelt anything that I can see ... is this something that is
known not to be doable?