Pro Site - Domain Mapping and checkout page

With Pro Sites enabled and a sub blog with a mapped domain using the WPMU Domain Mapping plugin.

Lets assume:
Main site that offers Pro Site upgrade =
Mapped domain which is a sub blog on the main site =

When you log in to the mapped domain and go to, the link to the Pro Sites upgrade page links to

NOT as one would expect it to go to:

In this case - the upgrade page is unavailble.