ERR_NGROK_SHIP_ENVIRONMENT_VAR_KEY_BAD_START: Environment variable KEY must not start with a digit - ngrok documentation

What does ERR_NGROK_SHIP_ENVIRONMENT_VAR_KEY_BAD_START mean?

ngrok returns ERR_NGROK_SHIP_ENVIRONMENT_VAR_KEY_BAD_START with this message:

Environment variable <KEY> must not start with a digit.

Where can I get more help with ERR_NGROK_SHIP_ENVIRONMENT_VAR_KEY_BAD_START?

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