I got an 'System.InvalidOperationException' and message 'Cannot save a PDF document with no pages.'

I generate a PdfDocument with ```TheArtOfDev.HtmlRenderer.PdfSharp.PdfGenerator.GeneratePdf(html, PdfSharp.PageSize.Letter)```, and the html has contents, but the PdfDocument is empty and cause a 'System.InvalidOperationException' when i save the document by ```pdf.Save(memoryStream)```