SQL & PL/SQL

Hi, i'm attempting to learn the mechanics behind collections and have been doing ok so far until i came across trying to use the index by varchar.

I'm using the following code:-

declare
  type address_lines_tab is table of varchar2(50)
    index by pls_integer;

  type address_tab is table of address_lines_tab
    index by varchar(40);

  type student_rec is record(
    first     varchar2(40),
    last      varchar2(40),
    address   address_tab
  );

  type student_tab is table of student_rec
    index by pls_integer;

  student   student_tab;
  i         number       := 0;
  v         varchar2(20);
  x         number       := 0;
begin
  student(1).first := 'mike';
  student(1).last := 'jones';
  student(1).address('home')(1) := 'the manor';
  student(1).address('home')(2) := 'london';
  student(1).address('home')(3) := 'w12 4kf';
  student(1).address('term')(1) := '5 university';
  student(1).address('term')(2) := 'plymouth';
  student(1).address('term')(3) := 'pl22da';
  student(2).first := 'helen';
  student(2).last := 'roots';
  student(2).address('home')(1) := '1 little lane';
  student(2).address('home')(2) := 'hull';
  student(2).address('home')(3) := 'h45 4fd';
  student(2).address('term')(1) := '3 university';
  student(2).address('term')(2) := 'plymouth';
  student(2).address('term')(3) := 'pl22da';

  for student_no in student.first .. student.last
  loop
    dbms_output.put_line(   'STUDNET NAME:'
                         || student(student_no).first
                         || ' '
                         || student(student_no).last);

    for add_type in
      student(student_no).address.first .. student(student_no).address.last
    loop
      dbms_output.put_line(student(student_no).address);
      for add_line in
        student(student_no).address(add_type).first .. student(student_no).address(add_type).last
      loop
        dbms_output.put_line(student(student_no).address(add_type)(add_line));
      end loop;
    end loop;
  end loop;
end;

the problem i'm having is in the second loop when i'm trying to display the 2 different types of address (ie home and term).

because its indexed by a varchar i'm getting the following error:-
ORA-06502: PL/SQL: numeric or value error: character to number conversion error

How do i get around this, when looping through associative arrays which are indexed by varchar?

many thanks

this is the output i would expect to see:-

STUDNET NAME:mike jones
HOME
the manor
london
w12 4kf
TERM
5 university
plymouth
pl22da
STUDNET NAME:helen roots
HOME
1 little lane
hull
h45 4fd
TERM
3 university
plymouth
pl22da