Setting up a Ground node using SpriteKit?

I've been designing a game where I want the game to stop as soon as the ball makes contact with the ground. My function below is intended to set up the ground.

class GameScene: SKScene, SKPhysicsContactDelegate {

     var ground = SKNode() 

     let groundCategory: UInt32 = 0x1 << 0
     let ballCategory: UInt32 = 0x1 << 1

     func setUpGround() -> Void {
            self.ground.position = CGPointMake(0, self.frame.size.height)
            self.ground.physicsBody = SKPhysicsBody(rectangleOfSize: CGSizeMake(self.frame.size.width, self.frame.size.height)) //Experiment with this
            self.ground.physicsBody?.dynamic = false

            self.ground.physicsBody?.categoryBitMask = groundCategory    //Assigns the bit mask category for ground
            self.ball.physicsBody?.contactTestBitMask = ballCategory  //Assigns the contacts that we care about for the ground

            self.addChild(self.ground)
        } 
    }

The problem I am noticing is that when I run the iOS Simulator, the Ground node is not being detected. What am I doing wrong in this case?

via Chebli Mohamed