extension for SKPhysicsContact crashing

I'm creating a game with SpriteKit, that has collision between 2 bodies. After setting up the bodies, I've implemented the didBegin(_contact:) moethod as shown below:

func didBegin(_ contact: SKPhysicsContact) {
    if contact.bodyA.categoryBitMask == 0 && contact.bodyB.categoryBitMask == 1 {
        gameOver()
    }
}

and it worked perfectly.

Later, while inspecrting the documentation for this method, I found the following:

The two physics bodies described in the contact parameter are not passed in a guaranteed order.

So to be on the safe side, I've extended the SKPhysicsContact class with a function the swaps the categoryBitMask between both bodies, as following:

extension SKPhysicsContact {

    func bodiesAreFromCategories(_ a: UInt32, and b: UInt32) -> Bool {
        if self.bodyA.categoryBitMask == a && self.bodyB.categoryBitMask == b { return true }
        if self.bodyA.categoryBitMask == b && self.bodyB.categoryBitMask == a { return true }
        return false
    }
}

The problem is that when the function gets called, the app crashes, and I get the following error:

2017-07-18 13:44:18.548 iSnake Retro[17606:735367] -[PKPhysicsContact bodiesAreFromCategories:and:]: unrecognized selector sent to instance 0x60000028b950

2017-07-18 13:44:18.563 iSnake Retro[17606:735367] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[PKPhysicsContact bodiesAreFromCategories:and:]: unrecognized selector sent to instance 0x60000028b950'