Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 20556)
+++ wp-includes/query.php	(working copy)
@@ -2304,8 +2304,10 @@
 			}
 			$q['author_name'] = sanitize_title_for_query( $q['author_name'] );
 			$q['author'] = get_user_by('slug', $q['author_name']);
-			if ( $q['author'] )
+			if ( $q['author'] && is_user_member_of_blog( $q['author']->ID ) )
 				$q['author'] = $q['author']->ID;
+			else 
+				$q['author'] = false;
 			$whichauthor .= " AND ($wpdb->posts.post_author = " . absint($q['author']) . ')';
 		}