Pro Sites: SSL Checkout Warning

Running Pro Sites with PP Pro on WPMS with subdomains.

Here's what happens. The person upgrades and after checkout when getting taken to their site a security warning pops up that the SSL certificate cannot be verified and asks if they want to make an exception. This is because the sub-domain isn't on the SSL cert and it's trying to send to the subdomain securely (why?)

What do we need to do to avoid this?

Thanks!