Button zindex overlay

[h3]

Maybe I'm doing something wrong, but I can't get buttons to behave as expected. They always overlay the menu pane.

class MainApp(App):
    nav = NavigationDrawer()

    def build(self):
        for k in SCREENS.keys():
            try:
                Builder.load_file('screens/%s.kv' % k)
            except IOError:
                print "[ERROR  ] [APP         ] Could not load screens/%s.kv" % k
                pass

        side_panel = BoxLayout(orientation='vertical')
        side_panel.add_widget(Label(text='Panel label'))
        side_panel.add_widget(Button(text='A button'))
        side_panel.add_widget(Button(text='Another button'))
        self.nav.add_widget(side_panel, 100)

        self.nav.add_widget(SCREENS['login'].get('screen')(name=SCREENS['login'].get('name')))

        return self.nav