Misleading type incompatibility message

flow-check

[rjkip:/tmp/flowtest]$ flow check  

/tmp/flowtest/game.js:5:11,16: ?Player
This type is incompatible with
  /tmp/flowtest/player.js:20:18,23: Player

Found 1 error

game.js

/* @flow */
var Player = require('./player');

class Game {
        winner: ?Player;

        constructor() {
                this.winner = null;
        }

        end(winner: Player) {
                this.winner = winner;
        }

        getWinner(): Player { // Return type is the true culprit
                return this.winner;
        }
}

var game = new Game();
game.end(Player.attacker());

player.js

/* @flow */
var attacker = new Player;
var defender = new Player;

class Player {
    static attacker(): Player {
        return attacker;
    }

    static defender(): Player {
        return defender;
    }

    /**
     * @private
     */
    constructor() {}
}

module.exports = Player;