ERR_NGROK_393: When specifying CIDRs to deny, you must specify at least one CIDR to allow - ngrok documentation

What does ERR_NGROK_393 mean?

ngrok returns ERR_NGROK_393 with this message:

When specifying CIDRs to deny, you must specify at least one CIDR to allow. Otherwise all traffic will be denied. Use 0.0.0.0/0 and ::0/0 to accept all traffic not explicitly blocked by your deny rules.

Where can I get more help with ERR_NGROK_393?

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