Error is null and borken up when checking on SelectSingleNode. Help me

var doc = htmldocument.DocumentNode.SelectNodes("//div[starts-with(@class, 'sense-block')]");
            foreach (var div in doc)
            {
                Assets.Dictionary Dic = new Assets.Dictionary();
                string vob = div.SelectSingleNode(".//span[@class='hw']").ToString();
                string word = div.SelectSingleNode(".//span[@class='pos']").InnerText.Trim();
                string pra = div.SelectSingleNode(".//span[@class='guideword']").InnerText.Trim();
                if(vob==null && worb==null && pra==null)
                {
                    Dic.Example = div.SelectSingleNode(".//span[@class='def-block']").InnerText.Trim();
                    Dic.Explaining = div.SelectSingleNode(".//span[@class='def']").InnerText.Trim();

                    Lst.Add(Dic);
               
                }
}

string vob = div.SelectSingleNode(".//span[@class='hw']").ToString();