Content can not be added to a PdfImportedPage - iTextSharp dll

Hey

I am using iTextSharp dll for merging, splitting and water marking pdf. It's working fine. When check earlier it's not working. when I debug the program I found following error.

Content can not be added to a PdfImportedPage.

at iTextSharp.text.pdf.PdfImportedPage.ThrowError()
at iTextSharp.text.pdf.PdfImportedPage.get_Duplicate()

Thanks in advance