Twilio: cannot rename subdomain null for SIP termination

问题

In relation to SIP registration, simply trying to add termination URI:

Termination URI

Configure a SIP Domain Name to uniquely identify your Termination SIP URI for this Trunk. This URI will be used by your communications infrastructure to direct SIP traffic towards Twilio. When you point your infrastructure toward this URI, Twilio uses a Geo DNS lookup to intelligently direct your traffic to our closest POP. Learn more about Termination Settings

I can add <foo>.pstn.twilio.com fine, but get an error or suggestion:

Please add at least one IP Access Control List or Credential List before saving your Termination settings.

Fair enough. Adding an IP Access Control List by clicking in the box generates:

Cannot rename subdomain null because the parent domain does not exist or this account doesn't own it.

I don't even know what to make of that. The IP Access Control List is already in the Twilio dashboard, I just selected a value already stored.

It's the same error message regardless of whether IP Access Control Lists or Credential Lists is used. In both cases, the values are configured and in the web dashboard.