help needed: _CameraDepthTexture returns a blank image?

reference used:
http://docs.unity3d.com/Documentation/Components/SL-CameraDepthTexture.html
http://docs.unity3d.com/Documentation/Components/SL-DepthTextures.html

using angrybot as a test case:

in InitLevelManager i add to Start():
Camera.main.depthTextureMode = DepthTextureMode.Depth;

and proceed to use the "Render Depth" shader as in reference.
modification to use float2 function instead as required in latest version.

the rendered texture is white. what did I miss?

I am trying to make a simple edge detector,
the plan was to use a sobel operator on the depth buffer,
thought it is simple and direct enough, guess not... derp.