ERR_NGROK_SHIP_PUBLISH_TARGET_REPOSITORY_PREFIX_ILLEGAL_CHARACTER: Docker Hub namespace PREFIX contains the illegal character CHAR - ngrok documentation

What does ERR_NGROK_SHIP_PUBLISH_TARGET_REPOSITORY_PREFIX_ILLEGAL_CHARACTER mean?

ngrok returns ERR_NGROK_SHIP_PUBLISH_TARGET_REPOSITORY_PREFIX_ILLEGAL_CHARACTER with this message:

Docker Hub namespace <PREFIX> contains the illegal character <CHAR>. Allowed: lowercase letters, digits, '-', and '_'.

Where can I get more help with ERR_NGROK_SHIP_PUBLISH_TARGET_REPOSITORY_PREFIX_ILLEGAL_CHARACTER?

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