Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 28206)
+++ src/wp-includes/functions.php	(working copy)
@@ -1079,20 +1079,19 @@
 	if ( $feed == '' || $feed == 'feed' )
 		$feed = get_default_feed();
 
-	$hook = 'do_feed_' . $feed;
-	if ( ! has_action( $hook ) )
+	if ( ! has_action( "do_feed_{$feed}" ) )
 		wp_die( __( 'ERROR: This is not a valid feed template.' ), '', array( 'response' => 404 ) );
 
 	/**
 	 * Fires once the given feed is loaded.
 	 *
-	 * The dynamic hook name, $hook, refers to the feed name.
+	 * The dynamic portion of the hook name, $feed, refers to the feed name.
 	 *
 	 * @since 2.1.0
 	 *
 	 * @param bool $is_comment_feed Whether the feed is a comment feed.
 	 */
-	do_action( $hook, $wp_query->is_comment_feed );
+	do_action( "do_feed_{$feed}", $wp_query->is_comment_feed );
 }
 
 /**