In iOS, using the google + sdk and specifically following their [sign-in documentation](https://developers.google.com/+/mobile/ios/sign-in)
I seem to be getting a nil userID, e.g. `[[GPPSignIn sharedInstance] userID]` is `nil`

The first time I authorize the application I get a valid userId, but the second time I enter my application expecting to see a valid user id, it's nil again.
When I check `[[GPPSignIn sharedInstance] trySilentAuthentication]` it returns true.

I'm following a very similar scenario to the one described in the [sample project](https://developers.google.com/+/mobile/ios/getting-started) .
What could be causing this?