Adding custom post type to category hides nav

When I add this code to my child theme functions.php (to include a custom post type), the navigation menu disappears from my category pages. Trying to figure out why.

add_filter('pre_get_posts', 'query_post_type');
function query_post_type($query) {
	if(is_category() || is_tag()) {
		$post_type = get_query_var('post_type');
	if($post_type)
			$post_type = $post_type;
	else
			$post_type = array('post','ebook');
		$query->set('post_type',$post_type);
	return $query;
		}
}