ERR_NGROK_439: The certificate_management_policy field must be null for domains owned by ngrok - ngrok documentation

What does ERR_NGROK_439 mean?

ngrok returns ERR_NGROK_439 with this message:

The certificate_management_policy field must be null for domains owned by ngrok.

Where can I get more help with ERR_NGROK_439?

Email support@ngrok.com and include the error code.