How can I solve this exception

object misValue = System.Reflection.Missing.Value;
           Excel.Application xlApp = new Excel.Application();
           xlApp.SheetsInNewWorkbook = SCANLA.Run.BusDetails.Count() ;
           Excel.Workbook xlWorkBook = xlApp.Workbooks.Add();
           xlApp.StandardFont = Constants.FONT_NAME;
           xlApp.StandardFontSize = Constants.FONT_SIZE;
           int SheetNumber = 0;
           string SheetName = null;

           try
           {
               string path = Properties.Settings.Default.OutputDirPath + "\\" + Constants.GT_EXCEL_NAME;
               if ((string)cmbNoBuses.SelectedItem == "2")
                   SheetName = "Bus 1,Bus 2";
               if ((string)cmbNoBuses.SelectedItem == "3")
                   SheetName = "Bus 1,Bus 2,Bus 3";
               if ((string)cmbNoBuses.SelectedItem == "4")
                   SheetName = "Bus 1,Bus 2,Bus 3,Bus 4";


               if (File.Exists(path))
                   File.Delete(path);

                   Excel.Worksheet xlWorkSheet = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(SheetNumber);

                   GenerateExcelSheets( xlWorkSheet, SheetName);
                   MessageBox.Show(Constants.GT_STATUS, Constants.GT_HEADING, MessageBoxButtons.OK, MessageBoxIcon.Information);


               xlWorkBook.SaveAs(path, Excel.XlFileFormat.xlWorkbookNormal, misValue, misValue, misValue, misValue, Excel.XlSaveAsAccessMode.xlShared, misValue, misValue, misValue, misValue, misValue);
               xlWorkBook.Close(true, misValue, misValue);
               xlApp.Quit();
           }
           catch (Exception ex)
           {
               throw new Exception(ex.Message);
           }
           finally
           {
               ReleaseObject(xlWorkBook);
               ReleaseObject(xlApp);
           }

An unhandled exception of type 'System.Exception' occurred in SCANLA.exe

Additional information: Invalid index. (Exception from HRESULT: 0x8002000B (DISP_E_BADINDEX))