Swift random point on screen for SKNode

Question

I am trying to generate a random point on the screen to create an SKSpriteNode:

let px : CGFloat = CGFloat(drand48())
let py : CGFloat = CGFloat(drand48())
let screenSize : CGRect = self.frame
let location = CGPoint(x: screenSize.width*px,y: screenSize.height*py)
let sprite = SKSpriteNode(imageNamed: "BlackCircle.png")

Although this generates points correctly within the frame, sometimes the node itself is not created because the location in the frame does not correctly translate to a location within the GameScene. I tested positions using:

print(touch.locationInNode(self))
print(touch.locationInView(self.view))

I know there should be some offset for the x and y coordinates, but I cannot figure them out. Am I using the wrong CGRect? I tried using:

UIApplication.sharedApplication().keyWindow?.frame

and

view!.frame

as screenSize instead, but I had the same problem. What CGRect should I use?