ERR_NGROK_SHIP_DOMAIN_APEX_UNSUPPORTED: PROVIDER cannot point a root domain at ngrok - ngrok documentation

What does ERR_NGROK_SHIP_DOMAIN_APEX_UNSUPPORTED mean?

ngrok returns ERR_NGROK_SHIP_DOMAIN_APEX_UNSUPPORTED with this message:

<PROVIDER> cannot point a root domain at ngrok. It serves no ALIAS, ANAME, or flattened CNAME record.

Where can I get more help with ERR_NGROK_SHIP_DOMAIN_APEX_UNSUPPORTED?

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