I want to take images rendered in Lightwave 9.3 into Photoshop CS2 for compositing. If:

1. I render objects with antialiased edges over a black background, and then
2. save out the alpha channel for use as a layer mask in Photoshop

I get a black fringe around the objects resulting from the semitransparent pixels of the antialiased edges revealing the black background in the base image.

How should I avoid this problem to get clean antialiased edges?

Thanks.