JWT - OAuth - Ruby - Boxr - 'cannot obtain token based on the enterprise configuration for your app'

I am attempting to implement JWT based authentication in a Rails app using the Boxr Ruby gem, but when requesting a user_token I receive an HTTP 400 response: "Cannot obtain token based on the enterprise configuration of your app."

I know my keys and secrets are set up properly because I was able to access a shared folder by using an enterprise token, but requesting a user token fails.

How should I troubleshoot this?