Single Product Pagination not working

the default wordpress next_post_link and previous_post_link does not work with the single product. When you set it in the same category TRUE, the product still navigates to a different category. The code belows work on other custom post type except for wp ecommerce.

<div class="previous-post-link">
                <?php previous_post_link('%link', '<< Previous Post', $in_same_term = true, $excluded_terms = '', $taxonomy = 'product_tag'); ?>
            </div>

            <div class="next-post-link">
                <?php next_post_link('%link', 'Next Post >>', $in_same_term = true, $excluded_terms = '', $taxonomy = 'product_tag'); ?>
            </div>

https://wordpress.org/plugins/wp-e-commerce/