trigger troubles

Why does this not work? It's a plpython function if that changes
anything

perversity=# \df for*
List of functions
Result data type | Schema | Name | Argument data types
------------------+------------+-------------------+---------------------
text | pg_catalog | format_type | oid, integer
"trigger" | public | foreign_key_check | text
(2 rows)

perversity=# create trigger f_foreign_key_check before insert or update
on f for each row execute procedure foreign_key_check ('f');
ERROR: CreateTrigger: function foreign_key_check() does not exist
perversity=#

James.