Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 15483)
+++ wp-includes/functions.php	(working copy)
@@ -1717,7 +1717,10 @@
 		wp_die( $message, '', array( 'response' => 404 ) );
 	}
 
-	do_action( $hook, $wp_query->is_comment_feed );
+	if ( $wp_query->is_comment_feed && $wp_query->is_singular && ( !current_user_can( 'read_private_posts' ) && 'private' == $wp_query->post->post_status ) )
+		include_once( get_404_template() );
+	else
+		do_action( $hook, $wp_query->is_comment_feed );
 }
 
 /**