[Solved] Export ASP.Net GridView using iTextSharp to PDF Error: The document has no pages

I am getting error below when i try to export GridView data to PDF.

An exception of type 'System.IO.IOException' occurred in itextsharp.dll but was not handled in user code

Additional information: The document has no pages.

Protected Sub ExportToPDF(ByVal sender As Object, ByVal e As EventArgs) Handles Button3.Click, Button3.Click, Button3.Click
    Using sw As New StringWriter()
        Using hw As New HtmlTextWriter(sw)
            'To Export all pages
            gvCustomers.AllowPaging = False
            Dim strConnString As String = ConfigurationManager.ConnectionStrings("SLISConnectionString").ConnectionString
            Using con As New SqlConnection(strConnString)
                Using cmd As New SqlCommand("SELECT * FROM Postserialnumberaccreg")
                    Using sda As New SqlDataAdapter()
                        cmd.Connection = con
                        sda.SelectCommand = cmd
                        Using dt As New DataTable()
                            sda.Fill(dt)
                            gvCustomers.DataSource = dt
                            gvCustomers.DataBind()
                            If dt.Rows.Count > 1 Then
                                '  gvCustomers.RenderControl(hw)
 
 
                                Dim sr As New StringReader(sw.ToString())
                                Dim pdfDoc As New Document(PageSize.A2, 10.0F, 10.0F, 10.0F, 0.0F)
                                Dim htmlparser As New HTMLWorker(pdfDoc)
                                PdfWriter.GetInstance(pdfDoc, Response.OutputStream)
                                pdfDoc.Open()
                                htmlparser.Parse(sr)
                                ' pdfDoc.NewPage()
                                pdfDoc.Close()
 
                                Response.ContentType = "application/pdf"
                                Response.AddHeader("content-disposition", "attachment;filename=GridViewExport.pdf")
                                Response.Cache.SetCacheability(HttpCacheability.NoCache)
                                Response.Write(pdfDoc)
                                Response.[End]()
                            Else
                                Dim time23 As String = "Specify Date Range and Click on Generate"
                                Dim script23 As String = "window.onload = function() { askConfirm('" & time23 & "'); };"
                                ClientScript.RegisterStartupScript(Me.GetType(), "askConfirm", script23, True)
                                Exit Sub
                            End If
                        End Using
                    End Using
                End Using
            End Using 
        End Using
    End Using
End Sub