Invoice shows "Sign Up" instead of "Pay"

I have a test membership setup that is 1 day free trial, then $8/day called "2-minute tracking"

I create a user, then log in with that user. Then i visit /memberships and select the 2-min tracking membership. No payment required because of the free trial I assume. Signup goes good, and the member gets attached to the membership, and the invoice get's generated (due 1 day in the future).

I look at the invoice (I'm currently logged in as the member), and instead of "Pay" it says "Signup" :

What's up with that?