如何解决错误：“ JSON写入中的类型无效（NSConcreteData）”

我正在尝试编写一些代码来验证我的iOS应用程序上的订阅。我正在关注本教程：http : //savvyapps.com/blog/how-setup-test-auto-renewable-subscription-ios-app/

它不在Swift 2.0中，因此我不得不转换一些代码，但是我在这一行遇到了麻烦：

let requestData = try! NSJSONSerialization.dataWithJSONObject(receiptDictionary, options: NSJSONWritingOptions.PrettyPrinted) as NSData!

当它到达该行时，它会显示以下错误消息：

由于未捕获的异常“ NSInvalidArgumentException”而终止应用程序，原因：“ JSON写入中的类型无效（NSConcreteData）”

这是整个功能：

func validateReceipt() {
    print("Validating")

    if let receiptPath = NSBundle.mainBundle().appStoreReceiptURL?.path where NSFileManager.defaultManager().fileExistsAtPath(receiptPath) {
        print("Loading Validation")
        let receiptData = NSData(contentsOfURL:NSBundle.mainBundle().appStoreReceiptURL!)
        print(receiptData)
        let receiptDictionary = ["receipt-data" :
            receiptData!.base64EncodedDataWithOptions([]), "password" : "placeholder"]
        let requestData = try! NSJSONSerialization.dataWithJSONObject(receiptDictionary, options: NSJSONWritingOptions.PrettyPrinted) as NSData!

        let storeURL = NSURL(string: "https://sandbox.itunes.apple.com/verifyReceipt")!
        let storeRequest = NSMutableURLRequest(URL: storeURL)
        storeRequest.HTTPMethod = "POST"
        storeRequest.HTTPBody = requestData

        let session = NSURLSession(configuration: NSURLSessionConfiguration.defaultSessionConfiguration())

        session.dataTaskWithRequest(storeRequest, completionHandler: { (data, response, connection) -> Void in

            if let jsonResponse: NSDictionary = try! NSJSONSerialization.JSONObjectWithData(data!, options:NSJSONReadingOptions.MutableContainers) as?
                NSDictionary, let expirationDate: NSDate = self.formatExpirationDateFromResponse(jsonResponse) {
                    print(expirationDate)
                    //self.updateIAPExpirationDate(expirationDate)
            }
        })
    }
}

我之前没有进行过应用内购买，因此对此我有点陌生。在此先感谢您的帮助！