PL/SQL type table index by binary_integer -- auto initialization??

Hi all.
(Oracle 9.2.0.1.0, Windows XP)

All documentation I've found states that you have to initialize a "record" in a PL/SQL type table (via extend) to use it.

However consider the following script:

---
DECLARE
  TYPE char_table IS TABLE OF VARCHAR2(1000);
  TYPE char_idx_table IS TABLE OF VARCHAR2(1000) INDEX BY
BINARY_INTEGER;
  v_t1 char_table;
  v_t2 char_idx_table;
  v_t1_2 char_table := char_table();
BEGIN
  FOR rec IN (
  SELECT rownum row_number, object_id, object_name FROM ALL_OBJECTS
WHERE rownum < 20 )
  LOOP


    /* This works! */

    v_t2(rec.object_id) := rec.object_name;


    /* Correct output: */

    dbms_output.put_line('Set '||rec.object_id||' to
'||v_t2(rec.object_id)||' ...');



    /* ORA-06531: Reference to uninitialized collection: */

    --v_t1(rec.object_id) := rec.object_name;



    /* ORA-06533: Subscript beyond count */

    -- v_t1_2(rec.object_id) := rec.object_name;

    v_t1_2.extend;


    /* ORA-06533: Subscript beyond count */

    -- v_t1_2(rec.object_id) := rec.object_name;



    /* This works (in combination with the extend above) */

    v_t1_2(rec.row_number) := rec.object_name;
  END LOOP;
END;
/
---

Can someone tell me where this behaviour is documented?

thanks!

best,
Martin