Custom post types & rewrite

Hi! I´ve been trying to make a custom post available under the URL mysite.com/news/year/postname.
Here is the code I come up with:

function create_post_types(){
	$args = array( 'label' => 'News',
				  'singular_label' => 'News',
				  'query_var' => true,
				  'rewrite' => array( 'slug' => 'news/%topic%' ) );
	register_post_type( 'news', $args );

	$args = array( 'hierarchical' => true,
		       'label' => 'Topics',
		       'rewrite' => false,
		       'query_var' => true );
	register_taxonomy( 'topic', 'news', $args );

	add_rewrite_tag( '%topic%', '(.+?)', 'post_type=news&topic=' );

	flush_rewrite_rules();
}
add_action( 'init', 'create_post_types' );

But it´s not working. If I try to access the post under the intended URL, it works; but it also works if I enter “asdf” for the category.
And, the permalinks created read “%topic%/test-post”, so the %topic%-tag doesn´t get replaced.

Anyone have an idea on what I did wrong?
Thanks for your help