openxml-issue-with-cellstyle

I'm trying to define a CellStyle within a completly empty spreadsheet. For the CellFormat I use the following code:

var workbookStylesPart = spreadsheetDocument.WorkbookPart.GetPartsOfType<WorkbookStylesPart>().First();
        //create default font for links
        Font font2 = new Font();
        Underline underline1 = new Underline();
        FontSize fontSize2 = new FontSize() { Val = 11D };
        Color color2 = new Color() { Theme = (UInt32Value)10U };
        FontName fontName2 = new FontName() { Val = "Calibri" };
        FontFamilyNumbering fontFamilyNumbering2 = new FontFamilyNumbering() { Val = 2 };
        FontScheme fontScheme2 = new FontScheme() { Val = FontSchemeValues.Minor };
        font2.Append(underline1);
        font2.Append(fontSize2);
        font2.Append(color2);
        font2.Append(fontName2);
        font2.Append(fontFamilyNumbering2);
        font2.Append(fontScheme2);

        workbookStylesPart.Stylesheet.Fonts.Append(font2);
        int fontId = workbookStylesPart.Stylesheet.Fonts.Count() - 1;

        CellFormat copy = (CellFormat)workbookStylesPart.Stylesheet.CellFormats.FirstOrDefault().Clone();
        copy.FontId = Convert.ToUInt32(fontId);

        workbookStylesPart.Stylesheet.CellFormats.Append(copy);

This all works fine, I can apply this cellformat to a cell and it is properly formatted, but as soon as I open try to add this to a CellStyle my Spreadsheet is broken and I can't open it anymore. This is how I add the CellStyle:

 workbookStylesPart.Stylesheet.CellFormats.Append(copy);
        int cellFormatId = workbookStylesPart.Stylesheet.CellFormats.Count() - 1;

        CellStyle cellStyle2 = new CellStyle() { Name = "linkformat", FormatId = Convert.ToUInt32(cellFormatId), BuiltinId = (UInt32Value)0U };

If I change the FormatId back to the value "1U" it works again. Does anyone have an idea how I can get this to work?