Can't register on to the free subscription plan

Hi

I have created a 'free' plan (called 'student') that costs nothing and lasts forever. There will never be payment for this plan.

Here's a screenshot:
http://dl.dropbox.com/u/120937783/Screenshots/Screen%20Shot%202013-09-13%20at%2012.49.24.png

However, when I try to test signup on the frontend, clicking 'subscribe' does nothing, it just reloads the page:
http://dl.dropbox.com/u/120937783/Screenshots/Screen%20Shot%202013-09-13%20at%2012.49.55.png

My expected functionality would be for it to redirect to a registration form?

Thanks