     }
     
     fileprivate func lookupPlayers() {
-        let playerIDs = match.players.map { $0.playerID } as! [String]
+        let playerIDs = match.players.map { $0.playerID } 
         
         GKPlayer.loadPlayers(forIdentifiers: playerIDs) { (players, error) in
             guard error == nil else {

    // MARK: Private functions
    
    @objc fileprivate func authenticationChanged() {
        if GKLocalPlayer.localPlayer().isAuthenticated && !authenticated {
            authenticated = true
        } else {
            authenticated = false

            }
            
            for achievement in achievements {
                self.achievements[achievement.identifier] = achievement
            }
            
            completion?()
