How do you get the "Current User" that is logged in via API?

I'm using the Authorization Code OAuth2 Flow, and after generating a Token, I would like to know which User logged in. Typically I'm used to API's having a /me style Endpoint that will return the current users information so that it can be displayed in the third party application, however all I see in the documentation is a /user API, and without specifying an ID or UserName, it always throws an error [500] - An unexpected error prevented your requested action from completing. The error has been logged for the administrator to review.

The backend shows the following error:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Telligent.Evolution.Rest.Resources.Users.UserController.Show(UserShowRequest request)