depth buffer sharing

I’m using a pbuffer and I would like to share zbuffer between my two contexts (original and pbuffer) to use it for depth of field computing :

-active pbuffer
-draw scene
-inactive pbuffer (orginal context)
-get pbuffer’s color buffer in a texture
-draw new scene with pbuffer’s depth buffer

I’ve found the constant WGL_SHARE_DEPTH_ARB for pixel format in pbuffer but when i use it my screen is dark and i don’t know why.