fetching table data into a table using cursor

I have a table called phonebook and it has two columns (firstName, LastName). I want to create a table of lastName index by firstName using cursor, and I wrote this code:

CREATE OR REPLACE PROCEDURE proc1 AS
    TYPE tableNames IS TABLE OF VARCHAR2(20) INDEX BY VARCHAR(20);
    v1 tableNames;
    v_firstName PHONEBOOK.FIRSTNAME%TYPE;
    v_lastName PHONEBOOK.LASTNAME%TYPE;
    CURSOR c_name IS SELECT FIRSTNAME, LASTNAME FROM PHONEBOOK;
BEGIN
    OPEN c_name;
    LOOP
        FETCH c_name INTO v_firstName, v_lastName;
        EXIT WHEN c_name%NOTFOUND;
        v1(v_firstName) := v_lastName;
    END LOOP;

    FOR idx IN v1.FIRST..v1.LAST 
    LOOP
        DBMS_OUTPUT.PUT_LINE (v1(idx));
    END LOOP;

    CLOSE c_name;
END;
/

It has been successfully compiled. When I run this procedure it should print lastNames which stored in the tableNames but it gave me an error:

ORA-06502 "PL/SQL: numeric or value error"
Cause: An arithmetic, numeric, string, conversion, or constraint error occurred. For example, this error occurs if an attempt is made to assign the value NULL to a variable declared NOT NULL, or if an attempt is made to assign an integer larger than 99 to a variable declared NUMBER(2).
Action: Change the data, how it is manipulated, or how it is declared so that values do not violate constraints.

Please help me to solve this problem

Not FOR, but WHILE. Also, I used cursor FOR loop as a source; easier to write & maintain.

SQL> create table phonebook (firstname varchar2(10), lastname varchar2(10));

Table created.

SQL> insert into phonebook
  2    select 'Little', 'Foot'    from dual union all
  3    select 'Mc'    , 'Donalds' from dual;

2 rows created.

SQL> create or replace procedure proc1 as
  2    type tablenames is table of varchar2(10) index by varchar2(10);
  3    v1 tablenames;
  4    idx varchar2(10);
  5  begin
  6    for cur_r in (select firstname, lastname
  7                  from phonebook
  8                 )
  9    loop
 10      v1(cur_r.firstname) := cur_r.lastname;
 11    end loop;
 12
 13    idx := v1.first;
 14    while idx is not null loop
 15      dbms_output.put_line(v1(idx));
 16      idx := v1.next(idx);
 17    end loop;
 18  end;
 19  /

Procedure created.

SQL> exec proc1;
Foot
Donalds

PL/SQL procedure successfully completed.

SQL>