function / trigger problem

Hello,

i've stumbled upon a problem with triggers and functions.

this function doesn't work.
It gives an error when creating the trigger: set_leentype() does not exist.

CREATE FUNCTION set_leentype (int4) RETURNS opaque AS '
BEGIN
NEW.type := $1;
RETURN NEW;
END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER is_boek
BEFORE INSERT ON boek
FOR EACH ROW
EXECUTE PROCEDURE set_leentype (1);

but if i use the same function and triggers, with the little
modification that i don't pass a parameter, it works fine!

does anyone know why this is and does anyone know a solution to this. (I
want to use the code with passing parameters).

thanx,
F.H.A. Masselink