order by SKU code – kind of working but not really – woocommerce

Hi there,

Thanks in advance for your help!

I am trying to order products by sku ascending on a shop page and custom tax page in woocommerce: http://www.playimports.com.au/la-grande-famille/

I am using the following code:

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

It kind of works but some of the products are out of order.

Does anyone have any ideas what I could try to make it order perfectly?

Thanks so much!
Sarah