tsearch trigger: function public.tsearch2(tsvector, text) does not exist

hi list

i'm trying to write a custom tsearch2 trigger that checks on update if the
column value is changed. here's what i did so far:

CREATE OR REPLACE FUNCTION "forum"."tsearch2_trigger_posts" () RETURNS
trigger AS
$body$
BEGIN
IF (TG_OP = 'UPDATE') THEN
IF (NEW.p_msg_clean != OLD.p_msg_clean) THEN
EXECUTE public.tsearch2(NEW.idxfti, NEW.p_msg_clean);
END IF;
RETURN NEW;
ELSIF (TG_OP = 'INSERT') THEN
EXECUTE public.tsearch2(NEW.idxfti, NEW.p_msg_clean);
RETURN NEW;
END IF;
RETURN NULL; -- result is ignored since this is an AFTER trigger
END;
$body$
LANGUAGE 'plpgsql' VOLATILE CALLED ON NULL INPUT SECURITY INVOKER;

CREATE TRIGGER "posts_ts_update" BEFORE INSERT OR UPDATE
ON "forum"."posts" FOR EACH ROW
EXECUTE PROCEDURE "forum"."tsearch2_trigger_posts"(idxfti, p_msg_clean);

unfortunately, the "EXECUTE public.tsearch2" part does not work:
Error while executing the query; ERROR: function public.tsearch2(tsvector,
text) does not exist at character 9 HINT: No function matches the given name
and argument types. You may need to add explicit type casts. QUERY: SELECT
public.tsearch2( $1 , $2 ) CONTEXT: PL/pgSQL function
"tsearch2_trigger_posts" line 4 at execute statement UPDATE forum.posts SET
p_msg_clean" = 'test' WHERE p_t_id = 4468

when using the public.tsearch2 function directly as a trigger, it works
fine:

CREATE TRIGGER "posts_ts_update" BEFORE INSERT OR UPDATE
ON "forum"."posts" FOR EACH ROW
EXECUTE PROCEDURE "public"."tsearch2"(idxfti, p_msg_clean);

when i'm trying to use EXECUTE PROCEDURE instead of EXECUTE within the
function, it won't compile:
ERROR: syntax error at or near "tsearch2" at character 19
QUERY: SELECT PROCEDURE public.tsearch2( $1 , $2 )
CONTEXT: SQL statement in PL/PgSQL function "tsearch2_trigger_news" near
line 4

what am i missing?

thanks,
thomas