DocumentNode in HtmlAgilityPack. Shouldn't SelectNodes return null?

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
}