Failed to apply number format with OpenXML in C #

I am trying to get a column in an excel document I am creating using OpenXML, which needs to be formatted in a friendly way for 12-bit UPC.

For this I am using the following code (based on this question ):

var sp = workbookpart.AddNewPart<WorkbookStylesPart>();
sp.Stylesheet = new Stylesheet {NumberingFormats = new NumberingFormats(), CellFormats = new CellFormats()};

var upcFormatting = new NumberingFormat {NumberFormatId = 164, FormatCode = "000000000000"};

var upcCellFormat = new CellFormat
                                {
                                        NumberFormatId = upcFormatting.NumberFormatId,
                                        FontId = 0U,
                                        FillId = 0U,
                                        BorderId = 0U,
                                        FormatId = 0U,
                                        ApplyNumberFormat = BooleanValue.FromBoolean(true)
                                 };
sp.Stylesheet.NumberingFormats.AppendChild(upcFormatting);
sp.Stylesheet.CellFormats.AppendChild(upcCellFormat);

sp.Stylesheet.NumberingFormats.Count++;
sp.Stylesheet.CellFormats.Count++;

var styleIndex = sp.Stylesheet.CellFormats.Count;

workbookpart.Workbook.Save();

      

        
        
        
      

    

The above code unfortunately creates a stylesheet that appears to be considered corrupted, looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<x:styleSheet xmlns:x="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
   <x:numFmts count="1">
     <x:numFmt formatCode="000000000000" numFmtId="164"/>
   </x:numFmts>
   <x:cellXfs count="1">
   <x:xf numFmtId="164" applyNumberFormat="1" xfId="0" borderId="0" fillId="0" fontId="0"/> 
   </x:cellXfs>
</x:styleSheet>

      

        
        
        
      

    

If anyone could please explain how I can get a valid stylesheet generated with a numbering format that allows 12 digits to be displayed using OpenXML only (you can't use any frameworks built around it like ClosedXML) I was would be very grateful.

Thank,