Created Issue: Tweaking substr causes errors [18459]

When tweaking the values of substr you can trigger an error by reducing the second value. So, for example I have it set at 10, 20 and that not quite right and i need 10,19, deleting the 0 from 20 will cause an exception. Obviously leaving it at 10,2 would make no sense but as part of the editing process it probably shouldn't throw.