Unable to get 1st day of month

I've to write a procedure to generate dynamic "values less than date" for adding new partitons to lot of tables but it doesn't generate 1st day of next month. I'm able to get the last day but when I try to add 1 day more it error out. Any idea?

This works fine.

  1  set serveroutput on SIZE 1000000
  2  set lines 150
  3  declare
  4     start_date date := last_day(to_date('01-JAN-2012','DD-MON-YYYY')) ;
  5  begin
  6     for ctr in 1 .. 12                     
  7     loop
  8             dbms_output.put_line('date less than '|| last_day(add_months(start_date,ctr-1)) || ' '|| to_char(last_day(add_months(start_date,ctr-1)),'MONYYYY')) ;