Can a MPMediaEntityPersistentID be cast to a value which can be saved to NSUserDefaults.standardUserDefaults?

Question

I am trying to save a [MPMediaEntityPersistentID] using NSUserDefaults.standardUserDefaults(); however, I cannot cast the MPMediaEntityPresitstentID to a type which can be saved.

Here is the code:

import Foundation
import AVFoundation
import MediaPlayer

class SaveFileArray 
{
    var songFileArray = [MPMediaEntityPersistentID]()

    init() {
    songFileArray = []
}

func saveFile(id: MPMediaEntityPersistentID) {
    let songID = id as NSString

    songFileArray.append(id)
    //Instead of printing lets save the MPMediaEntityPersistentID
    print(songFileArray[0])

    NSUserDefaults.standardUserDefaults().setObject(songFileArray, forKey: "time")
    NSUserDefaults.standardUserDefaults().synchronize()
}

func countSavedFiles() -> Int {
    return songFileArray.count
    }
}

Question:

1. Can anyone offer me a solution as to how I can save songID into NSUserDefaults?