FUNCTION ERROR

Hi.
i have a big problem with a function.

i create a type unidades
CREATE TYPE unidades AS (codigo numeric(3,0),
descripcion text);

and i create a function get_unidades
CREATE FUNCTION get_unidades(int4, int4, int4, int4) RETURNS
typeof
unidades AS '
SELECT cucoduni, cudesuni
FROM cumauni
WHERE cuempuni = $1 and
cuseduni = $2 and
cucaruni = $3 and
cuasiuni= $4
; ' LANGUAGE 'sql';

when execute this function

select * from get_contenidos(10,1,15,17,31,12)
as (codigo numeric(3,0),
descripcion text)

SAY

ERROR: ExecMakeTableFunctionResult: Invalid result from function returning
tuple

ANY IDEA