iOS 8 Swift and Facebook SDK v2.3 - User always logged out after relunch app

The application uses the login through facebook. Login works, but after closing the application, the user is not logged in. I tried to save the token with the following code but I get an error.

let defaults = NSUserDefaults.standardUserDefaults()
defaults.setObject(FBSDKAccessToken.currentAccessToken(), forKey: "FBTOKEN")

error:

Property list invalid for format: 200 (property lists cannot contain objects of type 'CFType')

Im using Xcode 6.2 and Facebook SDK 2.3