RSVP is not working: default attending even when user clicks on not attending

Hey! Strange problem since I've updated to the latest version of the plug-in.
When a user clicks on 'i'm not attending this event', he is RSVP'd on the attending list, even when the message is 'You are not attending'.

If you click a second time on 'I'm not attending', the user is on the correct not attending RSVP list.

Somebody knows what's the problem here?
Thanks for helping me out!

Koen