Exception in PdfFileSignature

Exception:

"System.InvalidCastException" at Aspose.Pdf.InteractiveFeatures.Forms.Signature

  Unable to cast object of type '\u0006Ž\u0013.†\u0013' to type '\u0006Ž\u0013.\u0010\u0013'.

Code:

            Aspose.Pdf.Document document = new Aspose.Pdf.Document(file);

            var pdfSign = new PdfFileSignature(document);

            var names = pdfSign.GetSignNames();

Aspose.Pdf 9.7.0