Set default header 2011 child theme

I’m just trying to set a default header image in a Twenty Eleven child theme. I’m getting “Constant HEADER_IMAGE already defined”
define('HEADER_IMAGE', get_stylesheet_directory_uri() . '/images/headers/default-deader.jpg');
is what I’m using. It works, but I want the error gone. I’ve searched the forums but didn’t find much and what I did find, didn’t get rid of the error.