SQL & PL/SQL :: Add A Not Null Constraint On A Collection For Testing?

I am trying to add a not null constraint on a collection for testing. It not allowed null values as below

pointers@ORCL> declare
2 type test_type is table of number not null;
3 t_test test_type;
4 begin
5 t_test := test_type(2,4,null); --null is being added
6 dbms_output.put_line('Type variable is initialized and the first element value is '||t_test(1));

[code]....

But what I observed is, i can add, null elements by using 'EXTEND' routine though 'not null' constraint is used for that type.

The below is the example.

pointers@ORCL> ed
Wrote file afiedt.buf
1 declare
2 type test_type is table of number not null;
3 t_test test_type;
4 begin
5 t_test := test_type(2,4);

[code]....