Depth Texture in Panda3D

Question

In Panda3D, I have the following code

self.manager = FilterManager(base.win, base.cam)

self.sceneTex = Texture("scene")
self.depthTex = Texture("depth")


self.quad = self.manager.renderSceneInto(colortex=self.sceneTex, depthtex = self.depthTex)

...   

When I run the above and enable view buffers (show-buffers #t), the "sceneTex" texture looks right. However, "depthTex" is always blank (all black) no matter where I move the camera to. Anyone know what's wrong?

Thanks