Unexpectedly found nil while unwrapping an optional value

 private let apiKey = ""//32 letters and numbers

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        let baseURL = NSURL(string: "https://api.forecast.io/forecast/\(apiKey)")
        let forecastURL = NSURL(string: "37.8267,-122.423", relativeToURL : baseURL)

        let sharedSession = NSURLSession.sharedSession()
        let downloadTask : NSURLSessionDownloadTask = sharedSession.downloadTaskWithURL(forecastURL!, completionHandler: { (location: NSURL!, response: NSURLResponse!, error: NSError!) -> Void in


            if (error == nil) {

                let dataObject = NSData(contentsOfURL: location)
                let weatherDictionary : NSDictionary = NSJSONSerialization.JSONObjectWithData(
                    dataObject!, options: nil, error: nil) as! NSDictionary

                println(weatherDictionary)



            }

        })
        downloadTask.resume()
    }