Here is an an actual code change that will work. Gizmo was close, but just needs to be a bit different and it needs to be general/default I've added it here as well so it should no longer break, unless someone just has a link to an avatar that doesn't exist.

Open up your cache_builders/featured_member.php script. Line 57, you'll see this:

Code
if (!$avatar) {

Change that, so it reads:

Code
if (!$avatar) {
        if (!$style_array['general']) {
                $style_array['general'] = "general/default";
        }