Question 415 Unsupported media type with CloudCodeAPI SubscriptionTokenProject()

I'm trying out the experimental API package, and getting a 415 Unsupported Media Type HTTP error when trying to listen for project events.

First I create the ServerClient

ApiService.CreateServerClient();

Then I set the service account credentials (is this even necessary for listening for project events?)

server.SetServiceAccount(id, key);

I log in

server.SignInWithServiceAccount(Application.cloudProjectId, environment);

But this returns a 415 Unsupported Media Type

server.CloudCode.SubscriptionTokenProject(Application.cloudProjectId);

I'd guess its something I"m doing wrong, but unsuppoted media tyhpe seems lower level than what I'd be able to mess up