我正在将表数据导出到excel，但出现以下详细信息错误： System.InvalidCastException: 'Unable to cast object of type 'System.String' to type 'OfficeOpenXml.ExcelColumn'.' . 我用的是excelpackage。
我的代码方法是：

public void ExportToExcel()
        {
//FECTH DATA FROM DB
            List<ExcellExport> excellExports = db2.MaintenanceWorkFlows.Select(
                x => new ExcellExport
                {
                    MaintenanceId = x.MaintenanceId,
                    RequisitionNo = x.RequisitionNo,
                    MaintenanceDate = x.MaintenanceDate,
                    MaintenanceFrequency = x.MaintenanceFrequency,
                    ContractNo = x.ContractNumber,
                    ContractorName = x.ContractorName,
                    MaintenanceStatus = x.MaintenanceStatus
                }).ToList();

从这里开始。我使用excel包导出数据

ExcelPackage.LicenseContext = LicenseContext.NonCommercial;
            ExcelPackage pck = new ExcelPackage();

            ExcelWorksheet ws = pck.Workbook.Worksheets.Add("ExpiredContracts");
            ws.Cells["A1"].Value = "Report";
            ws.Cells["B1"].Value = "Expired Contracts Report";

            ws.Cells["A1"].Value = "Date";
            ws.Cells["B1"].Value = string.Format("{0:dd MMMM yyy} at {0:H:mm tt}", DateTimeOffset.Now);

            ws.Cells["A4"].Value = "MainenanceId";
            ws.Cells["B4"].Value = "RequisitionNo";
            ws.Cells["C4"].Value = "MaintenanceDate";
            ws.Cells["D4"].Value = "MaintenanceFrequency";
            ws.Cells["E"].Value = "ContractNo";
            ws.Cells["F4"].Value = "ContractorName";
            ws.Cells["G4"].Value = "MaintenanceStatus";

            int rowStart = 10;

            foreach(var item in excellExports)
            {
                ws.Cells[string.Format("A{0}", rowStart)].Value = item.MaintenanceId;
                ws.Cells[string.Format("A{0}", rowStart)].Value = item.RequisitionNo;
                ws.Cells[string.Format("A{0}", rowStart)].Value = item.MaintenanceDate;
                ws.Cells[string.Format("A{0}", rowStart)].Value = item.MaintenanceFrequency;
                ws.Cells[string.Format("A{0}", rowStart)].Value = item.ContractNo;
                ws.Cells[string.Format("A{0}", rowStart)].Value = item.ContractorName;
                ws.Cells[string.Format("A{0}", rowStart)].Value = item.MaintenanceStatus;
            }
            ws.Cells["A:AZ"].AutoFitColumns();
            Response.Clear();
            Response.ContentType = "application/vnd.Openxmlformats.Officedocument-spreadsheetml.sheet";
            Response.AddHeader("content-disposition" ,"attachment:filename="+"ExpiredContractsReport.xlsx");
            Response.BinaryWrite(pck.GetAsByteArray());
            Response.End();

        }

我做错什么了？我看过代码，但没有解决问题。