tilemap in sprite kit - swift

Question

This is about drawing a TileMap in SpriteKit. I get the following error:
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attemped to add a SKNode which already has a parent: name:'(null)' texture:['nil'] position:{64, 0} size:{0, 0} rotation:0.00'
This is my GameViewController.swift:
import UIKit
import SpriteKit
class GameViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let scene = GameScene(size: CGSize(width: 1024, height: 768))
// Configure the view.
let skView = self.view as! SKView
skView.showsFPS = true
skView.showsNodeCount = true
/* Sprite Kit applies additional optimizations to improve rendering performance */
skView.ignoresSiblingOrder = true
/* Set the scale mode to scale to fit the window */
scene.scaleMode = .AspectFill
skView.presentScene(scene)
}
override func prefersStatusBarHidden() -> Bool {
return true
}
}
This is my GameScene.swift:
import SpriteKit
class GameScene: SKScene {
let dungeon = SKNode()
let readableMap: [[Int]] = [
[01, 00, 01, 01],
[01, 00, 01, 01],
[01, 00, 01, 01],
[00, 01, 00, 01]]
override init(size: CGSize) {
super.init(size: size)
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override func didMoveToView(view: SKView) {
/* Setup your scene here */
drawMap()
}
override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
/* Called when a touch begins */
for touch in (touches as! Set<UITouch>) {
let location = touch.locationInNode(self)
}
}
override func update(currentTime: CFTimeInterval) {
/* Called before each frame is rendered */
}
func drawMap() {
let map = readableMap.reverse()
let tiles = [[Tile]](count: map.count, repeatedValue: [Tile](count: map[0].count, repeatedValue: Tile()))
for i in 0..<map.count {
for j in 0..<map[0].count {
let tile = tiles[j][i]
tile.position = CGPoint(x: j * Int(tile.tileSize.width), y: i * Int(tile.tileSize.height))
dungeon.addChild(tile)
}
}
}
}
this is the class in Tile.swift:
import SpriteKit
class Tile: SKSpriteNode {
let tileSize: CGSize = CGSize(width: 64, height: 64)
let tileTexture: SKTexture = SKTexture(imageNamed: "o")
}
what could be the reason? Thanks to anyone, I'm still learning.