Can't put underscore in Workers Custom Domain

I’m trying to add a subdomain with an underscore in it as the custom domain for my worker, like worker_test.example.com but it tells me once I enter the underscore that this isn’t an active zone on my account which can be configured, however, it allows me to enter the same thing as a worker route which ends up working fine.

Not a huge deal since I can make it work with a route, but I’d prefer to use the custom domain if possible and it seems like a bug to me.

Having an underscore in an internet facing FQDN is problematic at best. I’d strongly recommend using a - instead.