Program crashes when I try to export to pdf file using CR for VS2010

I'm using VS2010, framework 4.0. I can view, print, and export the report to excel. When I click export, select pdf, enter a file name, and click save...I get the following message:

Value cannot be null.

Parameter name: window

I added Ludek's code to limit exports to pdf and excel to my JPTreportView.xaml.vb file:

Public Class JPTReportView

        Private Sub UserControl_Loaded(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles MyBase.Loaded

            reportViewer.ViewerCore.ReportSource = Me.DataContext.ReportSource

            reportViewer.ViewerCore.ToggleGroupTree = False

            reportViewer.ViewerCore.ToggleParameterPanel = False

            reportViewer.ViewerCore.ToggleSidePanel = False

            Dim exportFlags = (CrystalDecisions.Shared.ViewerExportFormats.PdfFormat Or CrystalDecisions.Shared.ViewerExportFormats.ExcelFormat)

            reportViewer.ViewerCore.AllowedExportFormats = exportFlags

        End Sub

    End Class