cache lookup failed

Hi,
I have this function:

CREATE FUNCTION add_one (integer) RETURNS integer AS '
BEGIN
RETURN $1 + 1;
END;
' LANGUAGE 'plpgsql';

from postgres doc.
When I try somethin like this:

SELECT add_one(4);

I get :

ERROR:  fmgr_info: function 20086: cache lookup failed

please tell me what it's rong.