Got Unrecognized selector -replacementObjectForKeyedArchiver: crash when implementing NSCoding in Swift

I created a Swift class that conforms to NSCoding. (Xcode 6 GM, Swift 1.0)

import Foundation

private var nextNonce = 1000

class Command: NSCoding {

    let nonce: Int
    let string: String!

    init(string: String) {
        self.nonce = nextNonce++
        self.string = string
    }
    
    required init(coder aDecoder: NSCoder) {
        nonce = aDecoder.decodeIntegerForKey("nonce")
        string = aDecoder.decodeObjectForKey("string") as String
    }
    
    func encodeWithCoder(aCoder: NSCoder) {
        aCoder.encodeInteger(nonce, forKey: "nonce")
        aCoder.encodeObject(string, forKey: "string")
    }
}

But when I call...

let data = NSKeyedArchiver.archivedDataWithRootObject(cmd);

It crashes gives me this error.

2014-09-12 16:30:00.463 MyApp[30078:60b] *** NSForwarding: warning: object 0x7a04ac70 of class '_TtC8MyApp7Command' does not implement methodSignatureForSelector: -- trouble ahead
Unrecognized selector -[MyApp.Command replacementObjectForKeyedArchiver:]

What should I do?

Although Swift class works without inheritance, but in order to use NSCoding you must inherit from NSObject.

class Command: NSObject, NSCoding {
    ...
}

Too bad the compiler error is not very informative :(