Selecting Nodes in xmldocument

Question

I am trying to call webservice and when hit search button to return value, the page throw error. I'm trying to parse data from calling API using xpath; 

Code:

protected string getattributevalue(string id, XmlDocument doc)
        {
            // Load the document and set the root element.
            XmlNode root = doc.DocumentElement;

            // Add the namespace.
            XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
            nsmgr.AddNamespace("go", "http://b.myserver.com/student/sign.xsd");
            String result;

            // Select and display the first node in which the author's
            try
            {
            XmlNodeList nodelist = root.SelectNodes("//signup", nsmgr);

           // Console.WriteLine(node.InnerXml);
            result = nodelist.Count.ToString();
            }
            catch (XPathException  ex)
            {
                Console.Write(ex.ToString());
                result = "Error: " + ex.ToString();
            }
            catch (NullReferenceException ex)
            {
                Console.Write(ex.ToString());
                result = "Error: " + ex.ToString();
            }

            return result;
        }

Error: System.NullReferenceException: Object reference not set to an instance of an object. at WebReg._Default.getattributevalue(String id, XmlDocument doc) in Default.aspx.cs:line 55  XmlNodeList nodelist = root.SelectNodes("//signup", nsmgr);