Index: wp-includes/feed.php
===================================================================
--- wp-includes/feed.php	(revision 5770)
+++ wp-includes/feed.php	(working copy)
@@ -116,7 +116,7 @@
 		$link = get_option('home') . '?feed=rss2&author=' . $author_id;
 	} else {
 		$link = get_author_posts_url($author_id, $author_nicename);
-		$link = $link . user_trailingslashit('feed', 'feed');
+		$link = trailingslashit($link) . user_trailingslashit('feed', 'feed');
 	}
 
 	$link = apply_filters('author_feed_link', $link);
@@ -134,7 +134,7 @@
 		$link = get_option('home') . '?feed=rss2&cat=' . $cat_ID;
 	} else {
 		$link = get_category_link($cat_ID);
-		$link = $link . user_trailingslashit('feed', 'feed');
+		$link = trailingslashit($link) . user_trailingslashit('feed', 'feed');
 	}
 
 	$link = apply_filters('category_feed_link', $link);