Mosaic screensaver with Aperture

[twig]

So far I'm finding Lion pretty good, but I'm having problems using the mosaic screensaver with my Aperture library. When the screensaver starts, I get a black screen with a "Looking for Pictures" message. It usually flickers to totally black, then the message reappears. After about 20 seconds, the screensaver begins, but as the mosaic zooms out there's a number of pictures or partial pictures which appear all black. My Aperture library has ~10 000 pictures, but it worked perfectly on Snow Leopard. Anyone else experience this or have any suggestions to fix?