Property.GetValue - Test for null breaks with inner exception of null reference?

I need to scan a bunch of Outlook '.msg' files and am using the MsgReader library to do so. (MsgReader Library Project on Github) I have written the following method which opens the message, reads through each of the properties and extracts any of those that are marked as 'string' to be added to a dictionary for returning to the caller. Here's the code:

    public Dictionary<string, string> GetMsgFileDetails(string filepath)
    {
        Dictionary<string, string> filedetails = new Dictionary<string, string>();
        //try
        {
            string fullpath = Path.Combine(SelectedFolder, filepath);
            var msg = new Storage.Message(fullpath);
            foreach (var prop in msg.GetType().GetProperties())
            {
                string pname = prop.Name;

                if (prop.GetValue(msg) != null)
                {
                    object pvalue = prop.GetValue(msg);
                }
                // We only want the strings
                if (prop.GetValue(msg) != null && (prop.GetValue(msg) is string))
                {
                    filedetails.Add(prop.Name, prop.GetValue(msg).ToString());
                }

            }
            return filedetails;
        }
        //catch (Exception ex)
        //{
        //    MessageBox.Show(ex.Message, "Error trying to load file");
        //    return null;
        //}
    }

 (the try-catch block is commented out to allow easier debugging).

It all seems to be working just fine but it hits one of the values which causes the

   (prop.GetValue(msg) != null)

line to break with a System Reflection TargetInvocationException . The exception details are as follows:

---

C:\Users\Brett\source\repos\FileMunger\FileMunger\ViewModels\FolderContext.cs:line 208
at FileThing.ViewModels.FolderContext.GetProperties(String filepath, Boolean showwindow) in C:\Users\Brett\source\repos\FileMunger\FileMunger\ViewModels\FolderContext.cs:line 159
at FileMunger.MainWindow.FileList_Selected(Object sender, RoutedEventArgs e) in C:\Users\Brett\source\repos\FileMunger\FileMunger\MainWindow.xaml.cs:line 121
at system.Windows.Controls.SelectionChangedEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
at System.Windows.Controls.ListBox.OnSelectionChanged(SelectionChangedEventArgs e)
at System.Windows.Controls.Primitives.Selector.InvokeSelectionChanged(List`1 unselectedInfos, List`1 selectedInfos)
at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
at System.Windows.Controls.Primitives.Selector.SelectionChanger.SelectJustThisItem(ItemInfo info, Boolean assumeInItemsCollection)
at System.Windows.Controls.ListBox.MakeSingleSelection(ListBoxItem listItem)
at System.Windows.Controls.ListBox.NotifyListItemClicked(ListBoxItem item, MouseButton mouseButton)
at System.Windows.Controls.ListBoxItem.HandleMouseButtonDown(MouseButton mouseButton)
at System.Windows.Controls.ListBoxItem.OnMouseLeftButtonDown(MouseButtonEventArgs e)
at System.Windows.UIElement.OnMouseLeftButtonDownThunk(Object sender, MouseButtonEventArgs e)
at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
at System.Windows.UIElement.OnMouseDownThunk(Object sender, MouseButtonEventArgs e)
at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
at System.Windows.Input.InputManager.ProcessStagingArea()
at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
at System.Windows.Application.RunDispatcher(Object ignore)
at System.Windows.Application.RunInternal(Window window)
at System.Windows.Application.Run(Window window)
at System.Windows.Application.Run()
at FileMunger.App.Main()

Inner Exception 1:
 NullReferenceException: Object reference not set to an instance of an object.

---

This seems to be due to the value of the prop.GetValue evaluating to null, but that is what I am trying to test for. I realise this might be getting into the depths of reflection where I am not an expert so if anyone has any ideas how to do this check I'd really appreciate it. Cheers.