NSKeyedArchiver breaking a Swift object

gistfile1.swift

// Crash reproduction
// Xcode 6.1 GM
// Create new project, Single View Application, Language: Swift
// Copy code below into ViewController.swift

import UIKit

class MyClass : NSObject, NSCoding {
    
    var myDictionary = [Int:Int]()
    
    override init() {
        
    }
    
    required init(coder aDecoder: NSCoder) {
        self.myDictionary = aDecoder.decodeObjectForKey("myDictionary") as [Int:Int]
    }
    
    func encodeWithCoder(aCoder: NSCoder) {
        aCoder.encodeObject(self.myDictionary, forKey: "myDictionary")
    }
    
    func setInt(value: Int, atIndex index: Int) {
        self.myDictionary[index] = value
    }
    
}

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
      
        let fileManager = NSFileManager.defaultManager()
        let documentDirectories = fileManager.URLsForDirectory(.DocumentDirectory, inDomains: NSSearchPathDomainMask.UserDomainMask) as [NSURL]
        
        let documentDirectory = documentDirectories[0]
        let fileName = documentDirectory.URLByAppendingPathComponent("file.archive")

        var myClass = MyClass()
        
        myClass.setInt(10, atIndex: 0)
        myClass.setInt(10, atIndex: 0)
        
        NSKeyedArchiver.archiveRootObject(myClass, toFile: fileName.path!)
        
        // CRASHOLA
        myClass.setInt(10, atIndex: 0)
        
    }
}