ORA-06531: Reference to uninitialized collection, Object Types with Inheritance!!!

Hi, i am using following code and getting "ORA-06531: Reference to uninitialized collection" ??

Please let me know...

Thanks,
Anil G

• Object Types Declarations

  ---

CREATE OR REPLACE TYPE t_a AS OBJECT(

           v_a VARCHAR2(100)
) NOT FINAL
/

CREATE OR REPLACE TYPE t_a_v AS VARRAY(100) OF t_a
/

CREATE OR REPLACE TYPE t_c AS OBJECT(

           v_d t_a_v
)
/

CREATE OR REPLACE TYPE t_b UNDER t_a (

           v_b VARCHAR2(100)
) FINAL INSTANTIABLE
/

• Sample Code to execute

  ---

  DECLARE --v_c t_c; v_1 t_a_v; v_2 t_b; BEGIN --v_c := t_c(t_a_v(t_b('1'))); --v_1(1) := t_b('1','1'); v_2 := t_b('Ha','Hb'); DBMS_OUTPUT.PUT_LINE( v_2.v_a||'----'||v_2.v_b); --v_1.EXTEND(1); v_1(0) := v_2; END;
  /