AVAudioPlayer unexpectedly found nil while unwrapping an Optional value

Question

I get the optional unwrapping error, but I can't see what is producing it. I checked the location of the mp3 file several times, I tracked down every line in the debug mode and it seems like it is getting a right file path in laughSound

Here is my code

import UIKit
import AVFoundation

class ViewController: UIViewController {

    var audioPlayer = AVAudioPlayer()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @IBAction func Laughing(sender: UIButton) {

        // Set the sound file name & extension
       var laughSound = NSURL(fileURLWithPath: NSBundle.mainBundle().pathForResource("Laughing", ofType: "mp3")!)

        // Play the sound
        audioPlayer = AVAudioPlayer(contentsOfURL: laughSound, error: nil)
        audioPlayer.prepareToPlay()
        audioPlayer.play()


    }

}

It gives me an error in this line

audioPlayer.prepareToPlay()

What should be the exact location of the audio file? I store it in the Supporting files and drag it to Xcode directly. Or what else can cause this problem? I've exhausted my search results.