ERR_NGROK_SHIP_PUBLISH_TARGET_REPOSITORY_PREFIX_BAD_START: Docker Hub namespace PREFIX must start with a lowercase letter or digit - ngrok documentation

What does ERR_NGROK_SHIP_PUBLISH_TARGET_REPOSITORY_PREFIX_BAD_START mean?

ngrok returns ERR_NGROK_SHIP_PUBLISH_TARGET_REPOSITORY_PREFIX_BAD_START with this message:

Docker Hub namespace <PREFIX> must start with a lowercase letter or digit.

Where can I get more help with ERR_NGROK_SHIP_PUBLISH_TARGET_REPOSITORY_PREFIX_BAD_START?

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