Null Ref if a Cell is null

[mstum]

var ws = new Worksheet("Testing");

ws.Cells[0,0] = "Foo";
ws.Cells[1,1] = null;

var wb = new Workbook();
wb.Add(ws);
wb.Save(@"test.xlsx");

NullReferenceException:

at Simplexcel.XlsxInternal.XlsxWriter.XlsxWriterInternal.HandleLargeNumbers(Worksheet sheet)
at Simplexcel.XlsxInternal.XlsxWriter.XlsxWriterInternal.Save(Workbook workbook, Stream outputStream, Boolean compress)
at Simplexcel.XlsxInternal.XlsxWriter.Save(Workbook workbook, Stream outputStream, Boolean compress)
at Simplexcel.Workbook.Save(Stream stream, Boolean compress)
at Simplexcel.Workbook.Save(String filename, Boolean compress)