รันโปรแกรมแล้วเจอ error Index and length must refer to a location within the string. ครับ แก้อย่างไรดี

//Read and show each line from file
        StreamReader sr = new StreamReader(HttpContext.Current.Server.MapPath("Model/test.xml"));
        string doc;
        string stemp;
        string text;
        string category;
        int begin, end;
        int numDoc = 0;
        while ((doc = sr.ReadLine()) != null)
        {
            //sw.WriteLine(doc);
            while (((stemp = sr.ReadLine()) != null) && (doc.IndexOf("</doc>") == -1))
            {
                doc += "\n" + stemp.ToLower();
            }
            if (doc.IndexOf("</doc>") == -1)
                break;
            numDoc++;
            if (numDoc % 100 == 0)
            {
                sw.WriteLine(".");
            }

            text = "";
            category = "-1";
            begin = end = -1;
            begin = doc.IndexOf("<category>") + 10;
            end = doc.IndexOf("</category>");

            if (begin < end)
            {
                category = (doc.Substring(begin, end)).Trim();
            }

            begin = end = -1;
            if (doc.IndexOf("<content>") != -1)
            {
                begin = doc.IndexOf("<content>") + 9;
                end = doc.IndexOf("</content>");
                if (begin < end)
                {
                    text = (doc.Substring(begin, end)).Trim(); <-------------- (Error)
                }
            }

            ////Write data
            sw.WriteLine("{" + formatText(text) + keywordTable.Count + "" + category + "}\n");
        }