Sorting Products by SKU

I want to sort products in Catalogue based on SKU. I am using following code(in functions.php), but it doesn’t seem to be working.

add_filter('woocommerce_get_catalog_ordering_args', 'am_woocommerce_catalog_orderby');
function am_woocommerce_catalog_orderby( $args ) {
    	$args['orderby'] = 'meta_value_sku';
	$args['order'] = 'asc';
	$args['meta_key'] = '';
}

Any help please?

http://wordpress.org/extend/plugins/woocommerce/