HtmlAgilityPack DocumentNode.SelectNodes returns null, shouldn't

I'm trying to scrape content from an example page using the HTML agility pack. The DocumentNode.SelectNodes is returning null for an XPath query when I think it shouldn't. Could someone tell me why? The code is:

HtmlDocument doc = new HtmlDocument();
string xpath = "//h1[@class='product-title fn']"; // note, it still returns 
                                                  // null even with "//div"
doc.OptionFixNestedTags = true;
HtmlNode.ElementsFlags.Remove("form");
HtmlNode.ElementsFlags.Remove("option");

HtmlNodeCollection coll = doc.DocumentNode.SelectNodes(xpath);

if (coll != null)
{
    // do stuff
}
else
{
    // not expecting it to be null unless no matches
}