OpenXML Spreadsheet: Excel gives an error, then fixes it, and it works. (WITH#)

I narrowed it down to:

       //aantal auto's
        Row aantalAutosRow = new Row();
        aantalAutosRow.RowIndex = Convert.ToUInt32((tblXML.Rows.Count) + 2);

        Cell aantalAutosCell = new Cell();
        aantalAutosCell.CellReference = getColumnName(1);
        aantalAutosCell.CellValue = new CellValue("Hoi");

        aantalAutosRow.Append(aantalAutosCell);
        sheetData.Append(aantalAutosRow);

This generates a cell after clicking the "Yes" button when Excel asks to fix the file (sheet.xml)

Full class:

            using System;
            using System.Collections.Generic;
            using System.Linq;
            using System.Text;
            using DocumentFormat.OpenXml;
            using DocumentFormat.OpenXml.Packaging;
            using DocumentFormat.OpenXml.Spreadsheet;
            using System.IO;
            using System.Xml;
            using System.Data;

            namespace XML_naar_Excel
            {
                class Excel
                {
                    public void convertToExcel(string padXml)
                    {
                        //maak dataset en vul met ingevoerde xml
                        DataSet dsXML = new DataSet();
                        dsXML.ReadXml(padXml);
                        DataTable tblXML = dsXML.Tables[2];

                        //opslaan
                        string padXlsx = Path.GetDirectoryName(padXml) + "\\voorraad " + DateTime.Now.Day + "-" + DateTime.Now.Month + "-" + DateTime.Now.Year + ".xlsx";

                        //spreadsheet
                        using (SpreadsheetDocument spreadsheetDocument = SpreadsheetDocument.Create(padXlsx, SpreadsheetDocumentType.Workbook))
                        {
                            WorkbookPart workbookPart = spreadsheetDocument.AddWorkbookPart();
                            WorksheetPart worksheetPart = workbookPart.AddNewPart<WorksheetPart>();
                            Workbook workbook = new Workbook();
                            FileVersion fv = new FileVersion();
                            fv.ApplicationName = "Microsoft Office Excel";
                            Worksheet worksheet = new Worksheet();
                            SheetData sheetData = new SheetData();

                            //styles
                            Style s = new Style();
                            WorkbookStylesPart stylesPart = spreadsheetDocument.WorkbookPart.AddNewPart<WorkbookStylesPart>();
                            stylesPart.Stylesheet = s.GenerateStyleSheet();
                            stylesPart.Stylesheet.Save();

                            //kolommen
                            Columns columns = new Columns();
                            for (int i = 1; i < tblXML.Columns.Count; i++)
                            {
                                Column column = new Column();
                                column.Min = Convert.ToUInt32(i);
                                column.Max = Convert.ToUInt32(i + 1);
                                int lengte = tblXML.Rows[13][i-1].ToString().Length;
                                if (lengte < 10)
                                {
                                    lengte = 10;
                                }
                                else if (lengte > 35)
                                {
                                    lengte = 35;
                                }
                                else
                                {
                                    lengte += 5;
                                }
                                column.Width = lengte;
                                column.BestFit = true;
                                columns.Append(column);
                            }
                            worksheet.Append(columns);

                            //header
                            Row header = new Row();
                            header.RowIndex = (UInt32)1;

                            foreach (DataColumn columnInDataTable in tblXML.Columns)
                            {
                                Cell headerCell = createTextCell(tblXML.Columns.IndexOf(columnInDataTable) + 1, 1, columnInDataTable.ColumnName);
                                headerCell.StyleIndex = 1;
                                header.AppendChild(headerCell);
                            }
                            sheetData.AppendChild(header);

                            // Add a row to the cell table.
                            for (int i = 1; i < tblXML.Rows.Count; i++)
                            {
                                Row row;
                                row = new Row() { RowIndex = Convert.ToUInt32(i) + 1 };

                                for (int j = 0; j < tblXML.Columns.Count; j++)
                                {
                                    Cell newCell = new Cell()
                                    {
                                        CellReference = getColumnName(i),
                                        DataType = CellValues.String,
                                        StyleIndex = 6,
                                        CellValue = new CellValue(tblXML.Rows[i][j].ToString()),
                                    };
                                    row.Append(newCell);
                                }
                                sheetData.Append(row);
                            }

                            //autofilter
                            string laatsteKolom = zoekLaatsteKolom(tblXML);
                            AutoFilter autoFilter = new AutoFilter();
                            autoFilter.Reference = "A1:" + laatsteKolom + "1";

                            //aantal auto's
                            Row aantalAutosRow = new Row();
                            aantalAutosRow.RowIndex = Convert.ToUInt32((tblXML.Rows.Count) + 2);

                            Cell aantalAutosCell = new Cell();
                            aantalAutosCell.CellReference = getColumnName(1);
                            aantalAutosCell.CellValue = new CellValue("Hoi");
                            //CellFormula berekenAantalAutos = new CellFormula();
                            //berekenAantalAutos.Text = "=COUNTA(A2:A" + tblXML.Rows.Count.ToString();

                            //aantalAutosCell.Append(berekenAantalAutos);
                            aantalAutosRow.Append(aantalAutosCell);
                            sheetData.Append(aantalAutosRow);

                            worksheet.Append(sheetData);
                            worksheet.Append(autoFilter);
                            worksheetPart.Worksheet = worksheet;
                            worksheetPart.Worksheet.Save();

                            Sheets sheets = new Sheets();
                            Sheet sheet = new Sheet();
                            sheet.Name = "Voorraad";
                            sheet.SheetId = 1;
                            sheet.Id = workbookPart.GetIdOfPart(worksheetPart);
                            sheets.Append(sheet);
                            workbook.Append(fv);
                            workbook.Append(sheets);

                            spreadsheetDocument.WorkbookPart.Workbook = workbook;
                            spreadsheetDocument.WorkbookPart.Workbook.Save();
                            spreadsheetDocument.Close();
                        }
                    }

                    //kolom naam (letter) ophalen
                    private string getColumnName(int columnIndex)
                    {
                        int dividend = columnIndex;
                        string columnName = String.Empty;
                        int modifier;

                        while (dividend > 0)
                        {
                            modifier = (dividend - 1) % 26;
                            columnName =
                                Convert.ToChar(65 + modifier).ToString() + columnName;
                            dividend = (int)((dividend - modifier) / 26);
                        }

                        return columnName;
                    }

                    private Cell createTextCell(int columnIndex, int rowIndex, object cellValue)
                    {
                        Cell cell = new Cell();

                        cell.DataType = CellValues.InlineString;
                        cell.CellReference = getColumnName(columnIndex) + rowIndex;

                        InlineString inlineString = new InlineString();
                        Text t = new Text();

                        t.Text = cellValue.ToString();
                        inlineString.AppendChild(t);
                        cell.AppendChild(inlineString);

                        return cell;
                    }

                    string zoekLaatsteKolom(DataTable tblXML)
                    {
                        string kolom;
                        kolom = getColumnName(tblXML.Columns.Count);
                        return kolom;
                    }
                }
            }

Can someone tell me what I am doing that is illegal for Excel? I think excel does SharedStrings when fixing it. I have never worked with SharedStrings, and I hope I do not need this.