Function removes main menu

I’m using the following function in function.php to display a custom post type in the main loop on a custom taxonomy page.

function namespace_add_custom_types( $query ) {
  if( is_tax() || is_tag() && empty( $query->query_vars['suppress_filters'] ) ) {
    $query->set( 'post_type', 'route' );
	  return $query;
	}
}
add_filter( 'pre_get_posts', 'namespace_add_custom_types' );

However when the taxonomy page is displayed the main menu doesn’t load. If I comment out the function the menu loads so I’m pretty sure the template is OK.

Any ideas?