SQL & PL/SQL

Given:

create or replace TYPE msgRecordTyp IS OBJECT (
r_msgid VARCHAR2(50),
r_trackid VARCHAR2(50)
);
/
create or replace TYPE msgTableArrayTyp AS
VARRAY(1000) OF msgRecordTyp;
/

declare
msgTab msgTableArrayTyp;
v_msgid varchar2(20) := 'msgid value';
v_trackid varchar2(20) := 'trackid value';
begin
msgTab := msgTableArrayTyp( msgRecordTyp(null, null) );
msgTab.extend;
msgTab(1).r_msgid := v_msgid;
msgTab(1).r_trackid := v_trackid;
dbms_output.put_line( msgTab(1).r_msgid );
--
msgTab.extend;
msgTab(2).r_msgid := v_msgid;
msgTab(2).r_trackid := v_trackid;
dbms_output.put_line(msgTab(2).r_msgid);
end;

I get:
pbrelin@DVDW.WORLD> /
msgid value
declare
*
ERROR at line 1:
ORA-06530: Reference to uninitialized composite
ORA-06512: at line 13

Notice the first assignment of the collection works. It fails on the second attempt....

Anyone know what I'm doing wrong?

Thanks in advance.
Paul