Index: wp-includes/link-template.php
===================================================================
--- wp-includes/link-template.php	(revision 19827)
+++ wp-includes/link-template.php	(working copy)
@@ -1104,6 +1104,10 @@
 
 	$current_post_date = $post->post_date;
 
+	// reset $in_same_cat if the post type doesn't support categories
+	if( ! is_object_in_taxonomy( $post->post_type, 'category' ) )
+		$in_same_cat = false;
+	
 	$join = '';
 	$posts_in_ex_cats_sql = '';
 	if ( $in_same_cat || ! empty( $excluded_categories ) ) {