overlaySKScene is not showing on ARSCNView

I'm using ARKit to create a game and I'd like to have an hud so I tried this:

let skScene = SightScene(size: self.view.bounds.size)
self.sceneView.overlaySKScene? = skScene
self.sceneView.overlaySKScene?.scaleMode = .aspectFill

Where sceneView is:

var sceneView: ARSCNView!

SightScene is:

class SightScene: SKScene {

    var circle: SKShapeNode!

    override init(size: CGSize) {
        super.init(size: size)

        circle = SKShapeNode(circleOfRadius: 40)
        circle.position = CGPoint(x: size.width/2, y: size.height/2 )
        circle.zPosition = 100
        circle.name = "Sight"
        circle.strokeColor = SKColor.black

        circle.glowWidth = 10.0
        circle.fillColor = SKColor.orange
        circle.physicsBody = SKPhysicsBody(circleOfRadius: 40)
        circle.physicsBody?.isDynamic = true

        self.addChild(circle)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }   
}

I wanted the shape node to spawn at the center of the screen. Please can someone help me ?

EDIT

I did this:

print(skScene)

I get the skScene informations

but when I do:

print(self.sceneView.overlaySKScene)    

I find nil, why ?