private void BindCountry()
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(Server.MapPath("countries.xml"));

            foreach (XmlNode node in doc.SelectNodes("//country"))
            {
                usrlocationddl.Items.Add(new ListItem(node.InnerText, node.Attributes["codes"].InnerText));
                //ddlCountry.Items.Add(new ListItem(node.InnerText, node.Attributes["code"].InnerText));
            }
        }

The above code am using for loading countries List from an xml file into a dropdown list. But while doing so am encountering a Null Reference error.

Object reference not set to an instance of an object.

Where in the code should I be changing, so that I can escape the error.