Excel 2007 Shared formula Error after setting cell formula to null

[AdPig]

I have a Excel 2007 file(xlsx) , C2 formula value=A2 * B2/100 ,if I select C2,drag From C2 to C4,then C3 to C4 Cell Value as follows:
C3=A3 * B3/100
C4=A4 * B4/100
at last i want clear C3 formula , I run follows code,but have error;

In a strange way, I Manual input C3 to C4 formula ,run follows code,but No error;

The Code is very sample,as follows:

       string excelFilePath = @"d:\test2.xlsx";
        IWorkbook workbook = null;
        using (FileStream fs = new FileStream(excelFilePath, FileMode.Open, FileAccess.Read))
        {
            workbook = new XSSFWorkbook(fs);
            ISheet sheet = workbook.GetSheetAt(0);
            ICell cell = sheet.GetRow(2).GetCell(2);
            cell.CellFormula = null;
        }
        FileStream sw = File.Create(excelFilePath);
        workbook.Write(sw);
        sw.Close();

errors,as follows:

test file:
TEST2.xlsx

ref infors:
NPOI Version : V2.5.6
.Net framework Version : 4.5

[tonyqus]

There is a removeFormula method in POI. I'll test if this works after migration

[tonyqus]

I found that the set-null code in SetCellFormula doesn't call XSSFSheet.OnDeleteFormula. It only calls XSSFWorkbook.OnDeleteFormula, which is not enough.

npoi/ooxml/XSSF/UserModel/XSSFCell.cs

Line 623 in 5fdb42c

((XSSFWorkbook)wb).OnDeleteFormula(this);