Hi all, i've a table of objects with a nested table. If i query for a ref of object inside that table i receive a null ref....why???

This is my situation:

Code:
CREATE OR REPLACE TYPE tCategoria AS OBJECT(
  nome VARCHAR2(100),
  nPremi INTEGER,
  minHandicap INTEGER,
  maxHandicap INTEGER,
  age INTEGER
);
/
 
CREATE OR REPLACE TYPE categorie_tab IS TABLE OF tCategoria;
/
 
CREATE OR REPLACE TYPE tGara AS OBJECT(
  nome VARCHAR2(100),
  DATA DATE,
  par INTEGER,
  reserved NUMBER(1),
  sponsor VARCHAR2(100),
  categorie categorie_tab,
  MEMBER FUNCTION classifica RETURN XMLTYPE
);
/
 
CREATE TABLE Gare OF tGara(
  nome NOT NULL,
  DATA NOT NULL,
  par NOT NULL,
  sponsor NOT NULL,
  reserved NOT NULL)
NESTED TABLE categorie STORE AS gara_categorie
/
 
SELECT * FROM gare g WHERE g.nome = 'Coppa del Presidente';
/
SELECT REF(g) FROM gare g WHERE g.nome = 'Coppa del Presidente';
The first query result is correct:
http://imageshack.us/f/441/resgh.png/

But with the second one i get a null ref:
http://imageshack.us/f/145/resquery.png/

Really, i don't understand why! :(

Thanks in advance and sorry for my english :P