ERR_NGROK_SHIP_ENVIRONMENT_VAR_KEY_ILLEGAL_CHARACTER: Environment variable KEY contains the illegal character CHAR - ngrok documentation

What does ERR_NGROK_SHIP_ENVIRONMENT_VAR_KEY_ILLEGAL_CHARACTER mean?

ngrok returns ERR_NGROK_SHIP_ENVIRONMENT_VAR_KEY_ILLEGAL_CHARACTER with this message:

Environment variable <KEY> contains the illegal character <CHAR>. Allowed: letters, digits, and '_'.

Where can I get more help with ERR_NGROK_SHIP_ENVIRONMENT_VAR_KEY_ILLEGAL_CHARACTER?

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