New Post: Cell is locked when I write to an existing sheet

                HSSFSheet ws = (HSSFSheet)wb.GetSheet(Maanden[maand-1]);
                var nextRow = StartRij-1;
                HSSFRow row = (HSSFRow)ws.CreateRow(nextRow);
                       row = (HSSFRow) ws.CreateRow(colum++);

                        row.CreateCell(rij++).SetCellValue(Datarow["Datum"].ToString());

                        row.CreateCell(rij++).SetCellValue(Datarow["Factuurnummer"].ToString());
                        row.CreateCell(rij).SetCellValue(Datarow["Omschrijving"].ToString());