Index: wp-admin/includes/screen.php
===================================================================
--- wp-admin/includes/screen.php	(revision 19628)
+++ wp-admin/includes/screen.php	(working copy)
@@ -21,7 +21,8 @@
 	static $column_headers = array();
 
 	if ( ! isset( $column_headers[ $screen->id ] ) )
-		$column_headers[ $screen->id ] = apply_filters( 'manage_' . $screen->id . '_columns', array() );
+		$column_headers[ $screen->id ] = array();
+	$column_headers[ $screen->id ] = apply_filters( 'manage_' . $screen->id . '_columns', $column_headers[ $screen->id ] );
 
 	return $column_headers[ $screen->id ];
 }