Positioning Nodes At Below the Bottom of the Screen (Spritekit)

Hello I'm trying to spawn bullets at the bottom of my screen to travel upwards but the current code that I have spawns the bullets at the top of the screen. I've tried making the height negative and nothing happened. Here's the code I'm working with, thanks.

let randomBulletPosition = GKRandomDistribution(lowestValue: -300, highestValue: 300)
    let position = CGFloat(randomBulletPosition.nextInt())
    bullet.position = CGPoint(x: position, y: self.frame.size.height + bullet.size.height)