string mx = sqlReader[0].ToString();出现错误了

           
各位编程爱好者，我是初学者，用C#写了下面这段程序，运行是出现了错误，麻烦各位给更正一下。谢谢!
            long mxh1;
            conn = new SqlConnection(Conn);
            conn.Open();
            SqlCommand sqlCmd4 = conn.CreateCommand();
             = "select max(lsh) as maxvlue from jsjb";  //lsh 是字段名，jsjb是表名，数据库连接是成功了的。
            sqlReader = sqlCmd4.ExecuteReader();
            string mx = sqlReader[0].ToString();//运行时这段语句出错了。
            long mxh = Convert.ToInt64(mx);
            mxh1 = mxh + 1;
            string mx1 = Convert.ToString(mxh1);
            conn.Close();