[Plugin: NextGEN Gallery] Nextgen Functions in category description

Hi,

I tried to include the NextGen Functions in the Cat. Description but did no succeed. This is my code in NextGen’s Sshortcode.php:

add_filter('the_content', array(&$this, 'convert_shortcode'));
		add_filter( 'term_description',array(&$this, 'convert_shortcode'));
		add_filter( 'category_description',array(&$this, 'convert_shortcode'));

I call the cat. Description this way:

$category_description = category_description();
						echo $category_description;

What could be the error?

http://wordpress.org/extend/plugins/nextgen-gallery/