HtmlAgilityPack used with linq to select nodes

Question

I basically try to use htmlagilitypack to parse a piece of html and use linq to put it into object for uses with other piece of my code. I've got below two code snippets where #1 uses linq and does not work, but #2 uses a for loop and works.

Two pieces of codes are nearly identical except for the usage of linq. In that sense, I guess I have missed something in linq that prevents me from making it work?

Appreciate if anyone can help me take a look

1. using linq - it does not work. res ends up being null

   res = from li in ul.Elements("li").Where(i => GetClass(i) != "titles-h")
   select new Post()
   {
       title = li.Element("h2").InnerText,
       // ....
   };
   

2. no linq - it works

   foreach (var li in ul.Elements("li"))
   {
       if (GetClass(li) != "titles-h")
           ress.Add(new Post()
             {
                 title = li.Element("h2").InnerText,
                // ....
             });
   }
   

implementation of GetClass

static string GetClass(HtmlNode n)
{
    if (n.Attributes["class"] != null)
        return n.Attributes["class"].Value;
    else
        return "";
}