Disallowed subdomains ignored

Issue Summary

Thanks for putting together this great module. I was previously using some ugly custom code to get the same functionality and I like this approach much better. Unfortunately, I've encountered a problem.

My site is at example.com. It has a taxonomy vocabulary called section with the following terms: main, a, and b. I would like nodes in section a to map to a.example.com/node/xxxx and nodes in section b to map to b.example.com/node/xxxx. However, I would like nodes in section main to map to example.com/node/xxxx, not to main.example.com/node/xxxx.

This is how I configured subdomain:

In my pathauto configuration, I set the Node path settings to [subdomain]/node/[nid].

Everything is working great, but when I create a node in section main, the alias points to
main.example.com/node/xxxx instead of example.com/node/xxxx. Is this incorrect behavior? What should I expect to happen if pathauto tries to create an alias to a disallowed subdomain?