Bug in Meteor client-side update

leaderboard.js

// See line 45 for the exception I'm trying to figure out

// Set up a collection to contain player information. On the server,
// it is backed by a MongoDB collection named "players."

Players = new Meteor.Collection("players");

function random_score () {
  return Math.floor(Math.random()*10)*5;
}

if (Meteor.is_client) {
  Template.leaderboard.sort_order = function () {
    return Session.equals("sort_order", 'name') ? {name: 1, score: -1} : {score: -1, name: 1};
  }

  Template.leaderboard.players = function () {
    return Players.find({}, {sort: Template.leaderboard.sort_order() });
  };

  Template.leaderboard.selected_name = function () {
    var player = Players.findOne(Session.get("selected_player"));
    return player && player.name;
  };

  Template.player.selected = function () {
    return Session.equals("selected_player", this._id) ? "selected" : '';
  };

  Template.leaderboard.events = {
    'click input.inc': function () {
      Players.update(Session.get("selected_player"), {$inc: {score: 5}});
    },
    'click input.dec': function () {
      Players.update(Session.get("selected_player"), {$inc: {score: -5}});
    },
    'click input.sort_by_name': function () {
      Session.set("sort_order", 'name');
    },
    'click input.sort_by_score': function () {
      Session.set("sort_order", 'score');
    },
    'click input.randomize_scores': function () {
      Players.find().forEach(function (player) {
        // Any idea why this throws the following exception?

        // Exception while simulating the effect of invoking '/players/update' undefined

        Players.update(player, {$set: {score: random_score}})
      });
    }
  };

  Template.player.events = {
    'click': function () {
      Session.set("selected_player", this._id);
    }
  };
}

// On server startup, create some players if the database is empty.
if (Meteor.is_server) {
  Meteor.startup(function () {
    if (Players.find().count() === 0) {
      var names = ["Ada Lovelace",
                   "Grace Hopper",
                   "Marie Curie",
                   "Carl Friedrich Gauss",
                   "Nikola Tesla",
                   "Claude Shannon",
                   "Sherpa Sherpa"];
      for (var i = 0; i < names.length; i++)
        Players.insert({name: names[i], score: random_score()});
    }
  });
}