Swift : JSON into string into NSData Error unrecognized selector sent to instance

I'm a little stuck here. I'm creating an NSMutableDictionnary that I'm converting into JSON with SwiftyJSON, and then I want to collect that json into a string, transform into NSData in order to send it into a post request.

Here is my code

    var dict:NSMutableDictionary = NSMutableDictionary()
    dict.setValue(flight.id, forKey: "id")
    dict.setValue(nil , forKey: "registration")
    dict.setValue(60 , forKey: "duration")
    dict.setValue(comment , forKey: "comment")

    var json = JSON(dict).rawValue

    let urlPath: String = "myawseomeurl"
    var url: NSURL = NSURL(string: urlPath)!

    var request: NSMutableURLRequest = NSMutableURLRequest(URL: url)

    request.HTTPMethod = "POST"

    let data = json.dataUsingEncoding(NSUTF8StringEncoding)

    request.timeoutInterval = 60
    request.HTTPBody=data
    request.HTTPShouldHandleCookies=false

    let queue:NSOperationQueue = NSOperationQueue()

    NSURLConnection.sendAsynchronousRequest(request, queue: queue, completionHandler:{ (response: NSURLResponse!, data: NSData!, error: NSError!) -> Void in

        var jsonResult: NSDictionary = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: nil) as NSDictionary

        })

And I get the following error:

[__NSDictionaryM dataUsingEncoding:]: unrecognized selector sent to instance 0x7fc9605e8ad0
2015-03-31 14:36:08.891 LearnToFly[18631:5221809] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSDictionaryM dataUsingEncoding:]: unrecognized selector sent to instance 0x7fc9605e8ad0'