Changing the Add To Cart button text in WooCommerce for items with Variations

The following lines of code don't seem to change the Add To Cart button text on my product page for an item with Variations. I'm running WooCommerce 2.5.5.

add_filter('variable_add_to_cart_text', 'my_custom_cart_button_text');

function my_custom_cart_button_text() {

        return __('Buy Now', 'woocommerce');

}

Would you happen to know what I'm missing?

via Chebli Mohamed