why this calculation results in NaN?

var player = {
    x : 10.0,
    y : 10.0,
    color : "blue",
    v : 5.0,
    size : 10,
    render : function (c) {
        c.fillStyle = this.color;
        c.fillRect(Math.round(this.x), Math.round(this.y), this.size, this.size);
    }
};

function update(delta) {
    player.x += player.v * (delta / 1000);
    console.log("px = " + player.x);
}

function mainLoop(timeStamp) {
    delta = timeStamp - lastFrameTime;
    lastFrameTime = timeStamp;
    /* measure frame rate */
    curFps = Math.round((1/delta)*1000);

    /* game logic */
    update(delta);
    render();

    requestAnimationFrame(mainLoop);
}

The update() function is called every frame when position of the player needs to be updated. value of delta is ~16.00 (time in ms). Why the value of player.x shows as NaN when printed?