There is a little problem with the language messages from form validation.
is_natural returns TRUE when number passed is in (0, 1, 2, ...)
is_natural_no_zero returns TRUE when number passed is in (1, 2, ...)
The messages are:
$lang['is_natural'] = "The %s field must contain only positive numbers.";
$lang['is_natural_no_zero'] = "The %s field must contain a number greater than zero.";
These 2 messages say the same thing, with different words. However, 0 is not a positive number, so the message from is_natural is wrong.
Fix issue #935 [ci skip]