Visible On/Off for a "menu"

[Murilo]

Hi... I'm trying to make visible or not pushing a key for a future menu...

import viz
viz.go()

menuScene = viz.addScene()

menuView = viz.addView()
menuView.setScene(menuScene)

menuWindow = viz.addWindow()
menuWindow.setClearMask(viz.GL_DEPTH_BUFFER_BIT)
menuWindow.setPosition(0,1)
menuWindow.setSize(1,1)
menuWindow.viewpoint(menuView)

#Create root node for menu quads
menu = viz.addGroup(scene = menuScene)
menu.translate(0,1.8,5)

#Create texture to apply to quads
tex = viz.addTexture('ball.jpg')

quad = viz.addTexQuad(parent=menu)
quad.texture(tex)
quad.alpha(0.7)

	
vizact.onkeydown( 'a', menuScene.visible(viz.ON) )
vizact.onkeydown( 's', menuScene.visible(viz.OFF) )



viz.add('gallery.ive')

When I push 'a' or 's', show this:
Traceback (most recent call last):
File "C:\Arquivos de programas\WorldViz\Vizard30/python\vizact.py", line 2667, in __onkeydown
self._callGroup(self.__keydownmap[key])
File "C:\Arquivos de programas\WorldViz\Vizard30/python\vizact.py", line 2619, in _callGroup
val = e.call(arg)
File "C:\Arquivos de programas\WorldViz\Vizard30/python\vizact.py", line 2470, in _callStatic
return self.func(*self.args,**self.kwargs)
TypeError: 'NoneType' object is not callable

What can I do!?
thanks,
Murilo