I get the following error when calling the package.Save(): Table Table1 Column Type does not have a unique name I gave the table a name, ensured that any null cells have a default empty type, but still cant find where its going wrong or how I can set the Column Type name that is not unique. Here s the code I use: public static bool ConvertToXlsx(string csvFilePath) { bool success = false; //we need an xlsx file path for the export and need to ensure the passed in file path is a CSV one var xlsxFilePath = Path.ChangeExtension(csvFilePath, "xlsx"); csvFilePath = Path.ChangeExtension(csvFilePath, "csv"); //convert the csv if (!string.IsNullOrWhiteSpace(xlsxFilePath) && !string.IsNullOrWhiteSpace(csvFilePath)) { try { using (ExcelPackage package = new ExcelPackage(new FileInfo(xlsxFilePath))) { //add a/another worksheet with datetime value so it doesn t clash with existing worksheets to the document ExcelWorksheet worksheet = package.Workbook.Worksheets.Add("Export_" + DateTime.Now.ToString("yyyy-dd-M--HH-mm-ss")); //starting from cell A1, load in the CSV file data with first row as the header worksheet.Cells["A1"].LoadFromText( new FileInfo(csvFilePath), new ExcelTextFormat { Delimiter = , }, OfficeOpenXml.Table.TableStyles.Light1, true); worksheet.Cells[worksheet.Dimension.Address].AutoFitColumns(); foreach (var cell in worksheet.Cells) { if (cell.Value == null) { cell.Value = ""; } } //save as xlsx package.Save(); success = true; } //attempt to delete the previously generated CSV file File.Delete(csvFilePath); } catch (Exception ex) { //if we cant delete the origionaly generated CSV file (used for the conversion), then return true if (ex.Message.Contains($"Access to the path {csvFilePath} is denied.")) { return true; } Console.WriteLine("Error: " + ex.Message); return false; } } return success; }