Created Unassigned: ClosedXML doc can't be open using Spire.Xls [9439]

I want to open a file created by ClosedXML.Excel and then user Spire.XLS to save it as a PDF.

The following code creates a simple Excel file using ClosedXML.Excel.
If I try to open the file using Spire.XLS I get a "Root element is missing" error.

If I open the file using Microsoft Excel and then save it, I can then open it using Spiral.

Any ideas why I can't open it in Spire without first saving it using Microsoft.Excel?



__var FilePath = @"c:\output.xlsx";
File.Delete(FilePath);


var workbook = new XLWorkbook();
var worksheet = workbook.Worksheets.Add("Data");
worksheet.Cell("A1").Value = "Hello World!";
workbook.SaveAs(FilePath);

// IF YOU OPEN AND SAVE THE FILE IN EXCEL THEN ALL WILL BE WELL, OTHERWISE YOUR'RE FUCKED
Spire.Xls.Workbook spWorkbook = new Spire.Xls.Workbook();
spWorkbook.LoadFromFile(FilePath);


var settings = new Spire.Xls.Converter.PdfConverterSettings() { FitSheetToOnePage = Spire.Xls.Converter.FitToPageType.ScaleWithSameFactor };
var pdf = Spire.Xls.PdfConvertionHelper.SaveExcelToPdf(spWorkbook, settings);

pdf.SaveToFile(Path.ChangeExtension(FilePath,"pdf"));__