Silverlight RIA AuthenticationService GetUser doesnt work through JSon

I've been trying to test out different RIA service calls through the Json endpoint开发者_Python百科 seem to have come up with an odd case. Using the default Business Application template, I can make calls to Login/Logout and various other custom methods I've put in, but for some reason GetUser wont work. It always returns HTTP 405, Method Not allowed

Does anyone have an idea why this is?

Using fiddler I cant see any difference between calling the JSon and the Binary endpoint yet they produce different results.