unable to evaluate expression because the code is optimized or a native frame c#

public void ExportToExcel(DataTable dt, string FileName)
      {
         
              System.Web.HttpContext.Current.Response.Clear();
              System.Web.HttpContext.Current.Response.Buffer = true;
              System.Web.HttpContext.Current.Response.AddHeader("content-disposition", "attachment;filename=" + FileName + DateTime.Now + ".xls");
              System.Web.HttpContext.Current.Response.Charset = "";
              System.Web.HttpContext.Current.Response.ContentType = "application/text";
              StringBuilder sb = new StringBuilder();
              sb.Append("<table cellSpacing='0' cellPadding='0' align='center' border='1'><tr>");

              for (int k = 0; k <= dt.Columns.Count - 1; k++)
              {
                  //add separator
                  //& sw.ToString() & "</table>")
                  sb.Append("<th align ='Left' width = '50px'>");
                  sb.Append(dt.Columns[k].ColumnName.ToString());
                  sb.Append("</th>");
              }
              sb.Append("</tr>");
              //append new line
              //sb.Append(Constants.vbCr + Constants.vbLf);
              for (int i = 0; i <= dt.Rows.Count - 1; i++)
              {
                  sb.Append("<tr>");
                  for (int k = 0; k <= dt.Columns.Count - 1; k++)
                  {
                      //add separator
                      string retval = dt.Rows[i][k].ToString();
                      sb.Append("<td align ='Left' width = '50px'>");
                      sb.Append(dt.Rows[i][k].ToString().Replace(",", ";"));
                      sb.Append("</td>");
                  }
                  sb.Append("</tr>");
                  //append new line
                  //sb.Append(Constants.vbCr + Constants.vbLf);
              }
              sb.Append("</table>");
              System.Web.HttpContext.Current.Response.Output.Write(sb.ToString());
              System.Web.HttpContext.Current.Response.Flush();
              //HttpContext.Current.ApplicationInstance.CompleteRequest();

              System.Web.HttpContext.Current.Response.End();
          }
         
          }
}