CHECK CONSTRAINT

I have 2 tables ART and MTF as follow:

create table art(
artid int4 primary key,
mtfid int4, -- can be null or foreign key with MTF

constraint ck_mtfid check(mtfid is null or fk_mtfid(mtfid))
);

create table mtf(
mtfid int4 primary key
);

create function fk_mtfid(int4) returns bool as '
declare
result int4;

begin
select mtfid into result from mtf where mtfid = $1;

if found then
return true;
else
return false;
end if;
end;
' language 'plpgsql';

The function works fine when I run it at the psql prompt, but when I tried
to insert some tuple in ART I have the following error:

"ERROR: init_fcache: Cache lookup failed for procedure 273568"

Best Regards,

Stephane.