ERR_NGROK_1623: KEY is not a valid HTTP header name because it contains at least one invalid character - ngrok documentation

What does ERR_NGROK_1623 mean?

ngrok returns ERR_NGROK_1623 with this message:

<KEY> is not a valid HTTP header name because it contains at least one invalid character.

Where can I get more help with ERR_NGROK_1623?

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