Pro Sites: Links to payment page not working

The links to the Pro payment page aren't working because they contain the pageID.

The link will look like this:
http://domain/?page_id=45089?bid=2995

Which will load to the correct payment page:
http://domain/prosite/?page_id=45089?bid=2995

I don't know where the page_id querystring parameter in comes from. The link will work if I delete the ?page_id=45089 from the url or change the ? before bid to &.

Is this a bug? Or did I mis configured something?

Thanks in advance,
Pieter