Illustrator Script Help

I'm getting an error at line 10 (see comment that says error). I'm not sure why this is happening. Someone please help.

if (!app.homeScreenVisible) {
	app.activeDocument.close(SaveOptions.SAVECHANGES);
}

var myDocument = app.documents.add();
myDocument.layers[0].name = 'BackgroundLayer';

for (var i = 0; i < 500; i++) {
	generateBackground(i);
	fileName = '~/Documents/tempFiles/test' + (i + 1); //error
	console.log(fileName);
	myDocument.exportFile(File(fileName), ExportType.PNG24);
}

function generateBackground(index) {
	var doc = app.activeDocument;
	var background = doc.pathItems.add();
	background.filled = true;

	var bgFillColour = new CMYKColor();
	var cyanNum = (index / (100 * 100 * 100)) | 0;
	var magentaNum = (index / (100 * 100)) | 0;
	var yellowNum = (index / 100) | 0;
	var blackNum = index % 100;

	bgFillColour.cyan = cyanNum;
	bgFillColour.magenta = magentaNum;
	bgFillCoulour.yellow = yellowNum;
	bgFillColour.black = blackNum;

	background.fillColor = bgFillColour;
	background.setEntirePath(
		[0, 0],
		[doc.width, 0],
		[doc.width, doc.height],
		[0, doc.height]
	);
}