Modal dialog under Unity does not block menu

An application, developed with wxWidgets (2.9.4), which works on Windows,
MacOSX & Ubuntu-Gnome is broken when it is executed under Unity.

Essentially, the application is a wxFrame with a menu, and a menu item
displays a dialog using ShowModal() (see below).

In Ubuntu-Gnome (and Win & MacOSX), the menu is disabled while the
dialog is displayed, which is the expected behavior. Under Unity (Ubuntu
 12.04.1 LTS & 12.10) the menu is not disabled while the dialog is
displayed: each subsequent click on the menu item displays a new dialog
window on top of the former dialog windows. Other active elements
(buttons) on the wxFrame are disabled while the dialog is displayed, as
they should be. The menu should be disabled as well.

Any way around this?

class myFrame: public wxFrame
{
...
}

void myframe::OnMenuitemClick( wxCommandEvent& event )
{
  myDialog* dp = new myDialog(this);

  dp->ShowModal();
}

class myDialog: public wxDialog
{
....
};