How to draw graphics to PDF using iText?

I'm trying to complete an example that draws a graph and writes it to a PDF, but I keep getting the error that the PDF has no pages. If I add something simple with document.add() after opening it works fine, I never see the graph. Here is my code:

Document document = new Document();
PdfWriter writer = new PdfWriter();
response.setContentType("application/pdf");
response.setHeader("Content-Disposition",
    " attachment; filename=\"Design.pdf\"");

writer = PdfWriter.getInstance(document, response.getOutputStream());

document.open();    
PdfContentByte cb = writer.getDirectContent();
Graphics2D graphics2D = cb.createGraphics(36, 54);
graphics2D.drawString("Hello World", 36, 54);
graphics2D.dispose();   
document.close();

Do I need to do something else to add the graphic to the document, or am I using the syntax incorrectly?