PL/SQL intervals

Could someone explain to me why the following creates an error? It only happens if the interval is over 100, however I thought that seeing as I declared days as day(9) this would be ok?

Could it be something to do with the return value of the function? AFAIK I cannot declare the return value to be:

return interval day(9) to second

or???????

here is the code:

declare

   2 b interval day(9) to second(9);
   3 function double_my_interval( a in interval day to second) return interval day to second

   4 is
   5 begin
   6 return a * 2;
   7 end;
   8 begin
   9 b :=((sysdate - 100) - sysdate) day(9) to second(9);   10
  11 dbms_output.put_line(b);
  12 dbms_output.put_line(double_my_interval(b));   13 end;
  14 /
declare
*
ERROR at line 1:
ORA-01873: the leading precision of the interval is too small ORA-06512: at line 12 Received on Mon Oct 25 2004 - 09:31:04 CDT