Why does this throw a Uncaught TypeError: Illegal invocation

Why does this throw a "Uncaught TypeError: Illegal invocation"?

function Game () {
    this.reqAnimFrame = window.requestAnimationFrame;

    this.gameLoop = function () {
        this.reqAnimFrame(this.gameLoop); // It's thrown on this line
    };
}

var game = new Game();
game.gameLoop();