Collection Help

I have a multi-level collection, indexed by VARCHAR2. After I fill it I try and traverse it and print out data and I'm getting an error. Can anyone see maybe what I am doing wrong?? Here is my code.

The actual error is referring to the first FOR index1 loop.

Thanks in advance!

TYPE order_rec IS RECORD (

  order_date         DATE,
  order_cnt          NUMBER,
  order_amt          NUMBER);

TYPE order_tab IS TABLE OF order_rec INDEX BY VARCHAR2(30);

TYPE registration_rec IS RECORD (

  date_registered      DATE,
  total_registered     NUMBER,
  order_data           ORDER_TAB);

TYPE registration_tab IS TABLE OF registration_rec INDEX BY VARCHAR2(30); v_order_tab REGISTRATION_TAB;

BEGIN
.
.

  FOR index1 IN v_order_tab.FIRST .. v_order_tab.LAST LOOP     v_string1 := TO_CHAR(v_order_tab(index1).date_registered,'Month YYYY') || ',' || v_order_tab(index1).total_registered;

    v_string2 := NULL;
    FOR index2 IN v_order_tab(index1).order_data.FIRST .. v_order_tab(index1).order_data.LAST LOOP

      v_string2 := v_string2 ||
TO_CHAR(v_order_tab(index1).order_data(index2).order_date,'Month') || ',' ||  

v_order_tab(index1).order_data(index2).order_cnt || ',' ||  

v_order_tab(index1).order_data(index2).order_amt;

    END LOOP;     UTL_FILE.PUT_LINE(v_file_id,v_string1 || ',' || v_string2);   END LOOP; ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character to number conversion error
ORA-06512: at line 80 Received on Wed Mar 14 2012 - 16:26:02 CDT