Why is my path position undefined in paper.js?

Question

The error I'm recieving from Chrome is: Uncaught TypeError: Cannot read property 'position' of undefined I don't understand why I can't access the position field of this path, as it is clearly defined in the documentation: http://paperjs.org/reference/path/#position
Here is my simple code:

paper.install(window);
var canvas = document.getElementById("letterCanvas");
window.onload = function()
{
    paper.setup("letterCanvas");

function Game(width, height)
{
    this.width = width;
    this.height = height;
    this.tps = 60;
    this.test = new Path.Rectangle(new Point(100,100), [50,100]);
    this.test.strokeColor = "black";
}

Game.prototype.tick = function()
{
    this.test.position = this.test.position.add(5,5);
}

function Activity(game)
{

}

view.onFrame = function()
{
    game.test.rotate(3);
}

var game = new Game(988,644);
__intervalId = window.setInterval(game.tick,1000/game.tps);
view.draw();
}

Answer 1

In your version game.tick invoked on window, use this

__intervalId = window.setInterval(function() { game.tick() } ,1000/game.tps);

Answer 2

Try game.tick() instead of game.tick