sgmlReader - why is my query finding no results

I'm trying to query an SGML file with sgmlReader, but the query is not finding any results. The code I have is:

string[] projectFiles = Directory.GetFiles(path, typeExtention, SearchOption.AllDirectories);

                foreach (string file in projectFiles)
                {


                    // setup SgmlReader
                    var sgmlReader = new Sgml.SgmlReader();
                    var myStringreader = new StringReader(File.ReadAllText(file));



                    sgmlReader.DocType = "HTML";
                    sgmlReader.WhitespaceHandling = WhitespaceHandling.All;
                    sgmlReader.CaseFolding = Sgml.CaseFolding.ToLower;
                    sgmlReader.InputStream = myStringreader;


                    XElement root = XElement.Load(sgmlReader);
                    IEnumerable<XElement> selectedElement =
                        from el in root.Elements("security")
                        where (string)el.Attribute("class") == "3"
                        select el; 

                    foreach (XElement el in selectedElement)
                        MessageBox.Show("text" + el);

                }

In the document i'm querying i clearly have the element and attribute <security class="3"> so i can't see why it's not finding it. I've debugged and it's looking in the right place for the file etc. Help appreciated. Thank you.