Sprite Kit Random Value never twice

I want to get a random value between 5 defined points, without getting the same result twice.

Here is a good explanation of how to avoid the same result twice: How to generate a random number in Swift without repeating the previous random number?

However, in my Code it doesn't work as it still is completely random, and I can't see why.

 func addWall(){

    let positions = [
        CGPoint(x: -120,y: -70),
        CGPoint(x: -50,y: -70),
        CGPoint(x: 10,y: -70),
        CGPoint(x: 70,y: -70),
        CGPoint(x: 120, y: -70),
    ]

    var previousPosition : Int?

    func randomPosition() -> Int {
        var randomPosition = Int(arc4random_uniform(UInt32(positions.count)))
        while previousPosition == randomPosition {
            randomPosition = Int(arc4random_uniform(UInt32(positions.count)))
        }
        previousPosition = randomPosition
        return randomPosition
    }

    Wall.position = positions[randomPosition()]
}