Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 4080)
+++ wp-includes/query.php	(working copy)
@@ -489,8 +489,12 @@
 	}
 
 	function set_404() {
+		$is_feed = is_feed();
+
 		$this->init_query_flags();
 		$this->is_404 = true;
+
+		$this->is_feed = $is_feed;
 	}
 
 	function get($query_var) {