Problem with setting up Aging periods definitions (AX 2012)

Hello,

could you plese help me with the following question:

I'm trying to set up the Aging periods definitions in Accounts Payable and can't set a period for 'not due' invoices (as in screenshot) - with unit 999 and interval 'unlimited':

I've got an error "You can only set up a unit less then 0".

What am I making wrong?

Thanks in advance!