Cache lookup failed with FUNCTION + TRIGGER

I have one table client :

create table client(
nocli int4,
....
);

create function client() returns opaque as '
begin
if new.nocli ISNULL then
raise exception "nocli null";
end if;
end;
' language 'plpgsql';

create trigger client before insert on client
for each row execute procedure client();

When I try to insert a tuple in client I have the following error:

"ERROR: fmgr_info: function 149857: cache lookup failed"

Where does this come from ? That's not the first time that I have this kind
of message!!

Please H E L P !!!!

RedHat 5.2
PostgreSQL 6.5.1

Regards,

Stephane FILLON