[ask] Return Query

hai all, i'm trying create function like this

CREATE OR REPLACE FUNCTION penjualan(idcb integer)
  RETURNS SETOF penjualan AS
$BODY$
BEGIN

IF ($1 IS NULL) THEN
  return query SELECT * FROM PENJUALAN;
ELSE
  return query SELECT * FROM PENJUALAN WHERE IDCABANG=$1;
END IF;

END;
$BODY$
  LANGUAGE 'plpgsql' VOLATILE
  COST 100
  ROWS 1000;

but this function is not work with this error :
ERROR:  structure of query does not match function result type
CONTEXT:  PL/pgSQL function "penjualan" line 6 at RETURN QUERY

on the time i try with other table and working well

thanks for your help