[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
satisfiesthe 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'::timestampwithout time zone)
 
(2 rows)

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