How to use DirectSelection ?

Hi, could somebody please explain to me how to use the DirectSelection module from direct.directtools ? I’ve set up some sample code but it just doesn’t work:

import direct.directtools.DirectSelection as DirectSelection
from direct.showbase.ShowBase import ShowBase

class PandaApp(ShowBase):
    def __init__(self):
        self.accept("mouse1", self.onLeftClick)
        model = loader.loadModel("Apartment02")
        self.selection = DirectSelection.SelectionRay()
    def onLeftClick(self):
        res = self.selection.pickGeom3D()
        print res
app = PandaApp()

The model is a simple house. I created an bounding box and tagged it with { barrier } in the .egg file. This is the output of

ModelRoot Apartment02.egg
  PandaNode Apartment02
    CollisionNode Collision (6 solids)
    GeomNode  (1 geoms: S:(MaterialAttrib TextureAttrib TransparencyAttrib))

The return value of pickGeom3D() is None and I just don’t know why. I guess something is wrong with my scene setup but I couldn’t find any documentation for DirectSelection. Could someone please explain to me what I’m doing wrong? I’m using Panda 1.7.2 built for Python 2.7.2 on Windows 7 SP1.

Thanks !