WooCommerce redirect subscription add to cart to checkout page

functions.php

<?php
add_filter('add_to_cart_redirect', 'custom_add_to_cart_redirect');
function custom_add_to_cart_redirect( $url ) {
  $product_id	= (int) $_REQUEST['add-to-cart'];
	if ( class_exists( 'WC_Subscriptions_Product' ) ) {
		if ( WC_Subscriptions_Product::is_subscription( $product_id ) ) {
			return get_permalink(get_option( 'woocommerce_checkout_page_id' ) );
		} else return $url;
	} else return $url;
}
?>