Free subscription plan not working.

I have a "free trial" subscription plan set to:

Finite
7 Days
$0.00

When I try to sign up for the free trial, it gets to the subscription page, but the subscribe button doesn't do anything. I can set it to $1.00 and it works, so it must be something with the price being $0.00.

How can I fix this?
http://yieldfitness.com/sign-up/