Проблема при рисовании на canvas (HTML5)

var cellWidth = 20;
var cellHeight = 20;

var canvasCWidth = 18;
var canvasCHeight = 4;

var canvasContainerId = 'c';

window.onload = function draw()
{
	var canvasWidth = cellWidth * (canvasCWidth + 6);
	var canvasHeight = cellHeight * (canvasCHeight + 8);
	var canvas = document.getElementById(canvasContainerId);
	
	canvas.style.width = canvasWidth + 'px';
	canvas.style.height = canvasHeight + 'px';
	
	var ctx = canvas.getContext('2d');
	
	drawBG(ctx);
	
}

function drawBG(ctx)
{
	ctx.beginPath();
	
	for (i = 1; i <= canvasCWidth; i++)
	{
		for (j = 1; j <= canvasCHeight; j++)
		{
			ctx.strokeRect(cellWidth * i, cellHeight * j, cellWidth, cellHeight);
		}
	}
	
	ctx.closePath();
}