Domain mapping with Pro sites

I have Pro Sites & the Domain Mapping plugins installed. Both are now set-up and working great.

I then added a mapped domain to my main site (eg the one users sign up for a new blog via pro sites on). The new mapped domain works fine.

However there is an error in the Pro Sites checkout when the mapped domain is used. On the original URL the checkout page displays & works correctly. On the mapped domain the checkout page is blank.

Any ideas why this might be? (eg do the two plugins work together only if the main domain isn't mapped? Or should this work and there's some kind of error?)