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

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C#

MS-Excel

OpenXML

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 the Cell after I click yes when Excel prompts to fix the file (sheet.xml)

Complete 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'm doing that is illegal to Excel? I think excel makes SharedStrings when mending it. I never worked with SharedStrings and I hope I don't have to.