Domain Mapping + Pro Sites + SSL

I'm doing some testing with Pro Sites. I have Pro Sites and Domain Mapping enabled and everything is using SSL (wildcard). My subsites do fine; they show as HTTPS.

However when I go to my pricing/features table page and select a plan, it pops the fields for my site name and URL (as it should). When I submit those, however, it just spins... when I inspect, I see the dreaded "[site] was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint ''. This request has been blocked; the content must be served over HTTPS."

When I disable domain mapping, it works fine.

As it happens, I have no mapped domains on this site yet... surely that can't be a factor.

I've tried about everything I can think of and no luck. Are there any known issues along these lines? I've done a lot of searching and haven't found anything.