WooCommerce：在其类别中随机排序产品

I want that all my products be sorted randomly, inside each of their categories. But WooCommerce doesn't allow this.

add_filter( 'woocommerce_get_catalog_ordering_args', 'custom_catalog_ordering_args', 20, 1 );
function custom_catalog_ordering_args( $args ) {

    $args['orderby'] = 'rand';
    return $args;
}

What's wrong please ?