Pro Sites Free Level

Hi,

When I try to add a new ' Free ' level with ' 0 ' as $ pricing, I get the following error: You must enter a price for at least one payment period.
I've read on other threads that creating a Free level is possible with setting the price as ' 0 ', so why can't I do it? I need to show the free level in the pricing table next to the Pro level but can't seem to find out how!

Thank you.