iOS Google oAuth用户ID始终为零

Question

每当Google返回＆＃34; finishedWithAuth＆＃34; userID变量始终为nil

这是我的代码

override func viewDidLoad() {
    super.viewDidLoad()

    // Configure the Google+ Sign In
    var signIn = GPPSignIn.sharedInstance()
    signIn.scopes = ["profile"];
    signIn.shouldFetchGooglePlusUser = true
    signIn.clientID = kClientId
    signIn.shouldFetchGoogleUserEmail = true
    signIn.shouldFetchGoogleUserID = true

    signIn.trySilentAuthentication()
    signIn.delegate = self
}

func finishedWithAuth(auth: GTMOAuth2Authentication!, error: NSError!) {
    if(error == nil && auth != nil){
        println(auth.userID) // always nil
        println(auth.userEmail) // always contains user's email
    }else{
        println("Recieved error \(error) and auth object \(auth)")
    }
}

我无法看到我做错了什么