iTextSharp - use a custom image for a list bullet symbol

Question

I'm trying to use a custom image as a bullet symbol in a list, but it's not showing up (there's empty space where the bullet should be). I get no errors and the PDF loads fine; the bullet just doesn't show.

Relevant code:

// the 'content' var is a PdfContentByte 
// passed to the function this code is in

var imagePath = Server.MapPath("~/Bullet.png");
var bullet = iText.Image.GetInstance(imagePath);

list.Symbol = new Chunk(bullet, 0, 0);
foreach (var item in items)
    list.Add(new iText.ListItem(item, font));

var text = new ColumnText(content);
text.SetSimpleColumn(120, 390, 670, 100, 36, Element.ALIGN_LEFT);
text.AddElement(list);
text.Go();

What am I doing wrong?