Created Unassigned: Using SaveAs on Workbook not working [21130]

I have opened a file in Excel but when I try to use the workbook SaveAs method no errors are generated but the file does not get saved.

(I have Excel v15)

if (excelVersion >= 12.0)
book.SaveAs(useFilename, XlFileFormat.xlWorkbookDefault);
else
book.SaveAs(useFilename);