Free subscription?

How do you create a free subscription? I choose between finite, indefinite or serial, the period of time and the price- which I want at 0, but don't have that option. If I leave it blank it doesn't save. (Adding another one in the "drop here", doesn't seem to help either.) Help. Thanks

(Looking at your website screenshots, made it look I could do this. https://premium.wpmudev.org/view-screenshot/1347109618_membership_sample_subscription.png)