No information return into completion of logInWithMethods



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 {
    } else if let error = error {
    } else {
        //What's happened?