No information return into completion of logInWithMethods

ios
swift

#1

I’m using following code.

I will get two nils as session and error even thought valid twitter account is set in Settings app of iOS.
I am not sure what is happened.

Would you help me?

logInWithMethods(.SystemAccounts) { session, error in
    if let session = session {
        //succeeded
    } else if let error = error {
        //error
    } else {
        //What's happened?
    }
}