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) RETURNStypeofunidades AS
'SELECTcucoduni, cudesuniFROM cumauniWHERE 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