Cache lookup error

Hi i keep getting this error but i definitely dont understand or know how to
correct it...

SELECT * FROM BUSCabLista('0', '000081');
ERROR: cache lookup for type 0 of cabezalista.........pg.dropped.41........
failed
WARNING: plpgsql: ERROR during compile of buscablista near line 0
WARNING: plpgsql: ERROR during compile of buscablista near line 0
ERROR: cache lookup for type 0 of cabezalista.........pg.dropped.41........
Failed

The function looks like this:

DECLARE
                Codigo ALIAS FOR $1;
                Libro ALIAS FOR $2;
                res cabezalista%ROWTYPE;
        BEGIN

                FOR res IN select * from cabezalista where idlista like CAST
(Codigo AS CHAR(20)) and idlibro like CAST(Libro AS CHAR(6)) LOOP
                        RETURN NEXT res;
                END LOOP;
                return;
        END;

Thnks!

JuanF