How to properly use `GKLocalPlayer.anonymousGuestPlayer` for a computer player

Hi, I am trying to create an anonymous guest player to act as a computer player to fill in for an unfilled multiplayer slot.

Invoking the register method after the user has authenticated causes an exception to be thrown.

[GKAnonymousGuestPlayerInternal registerListener:]: unrecognized selector sent to instance

class ComputerPlayer: NSObject, GKLocalPlayerListener {
    let player: GKLocalPlayer

    override init() {
        self.player = GKLocalPlayer.anonymousGuestPlayer(withIdentifier: "local_anon_guest")
        super.init()
    }

    func register() {
        self.player.register(self) // <-- this throws "-[GKAnonymousGuestPlayerInternal registerListener:]: unrecognized selector sent to instance"
    }

    func player(_ player: GKPlayer, receivedTurnEventFor match: GKTurnBasedMatch, didBecomeActive: Bool) {
        ...
    }

}

There is very little documentation on its proper use, and after hours of searching I have not been able to find a single other example of the anonymous guest player API being used.

Can someone guide me on the proper use of this API?