Inserting formula giving error.. Excel found unreadable content in “ab.xlsx”. Do you want to recover the…

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my requirement to is to insert formula for a cell. i am using below method to insert formula. And its inserting formula corectly and formula working fine. but when i insert formula my excel file got corrpted and showing the message "Excel found unreadable content in "exceltemplate.xlsx". Do you want to recover the contents of..........". I searched lot,but not getting resolved. Please help to resolve this..

public void InsertFormula(string filepath, string SheetName, string strCellIndex, string strFormula)
{
    using (SpreadsheetDocument document = SpreadsheetDocument.Open(filepath, true))
    {
        IEnumerable<Sheet> sheets = document.WorkbookPart.Workbook.Descendants<Sheet>().Where(s => s.Name == SheetName);
        if (sheets.Count() == 0)
        {
            // The specified worksheet does not exist.
            return;
        }
        WorksheetPart worksheetPart = (WorksheetPart)document.WorkbookPart.GetPartById(sheets.First().Id);
        Worksheet worksheet = worksheetPart.Worksheet;
        SheetData sheetData = worksheet.GetFirstChild<SheetData>();

        Row row1 = new Row()
        {
            RowIndex = (UInt32Value)4U,
            Spans = new ListValue<StringValue>()
        };

        Cell cell = new Cell() { CellReference = strCellIndex };
        CellFormula cellformula = new CellFormula();
        cellformula.Text = strFormula;
        cell.DataType = CellValues.Number;
        CellValue cellValue = new CellValue();
        cellValue.Text = "0";
        cell.Append(cellformula);
        cell.Append(cellValue);
        row1.Append(cell);

        sheetData.Append(row1);
        worksheet.Save();
        document.Close();
    }
}

And i am calling that method like below

InsertFormula(filepath, SheetName, "D15","=SUM(D9:D11)");
InsertFormula(filepath, SheetName, "D16", "=IF(D15-D6<0,0,D15-D6)");