'Create an account' lightbox appears after submitting order

Hey,

We’re just testing some bookings on our new website here: http://containment.zone/store/products/booking/

We’ve noticed that when we have submitted our order in the checkout, a ‘create an account’ lightbox appears. This is really confusing and suggests people have to create an account to complete the order. We know they don’t, because you can just click the ‘x’ in the top right corner of the lightbox – and behind it shows that the order is confirmed.

Please can tell us what to do so that this lightbox doesn’t show?

Thanks, Phil.