Add to Cart Not Functioning

Recently upgraded and the “Add to Cart” button doesn’t seem to be working. I just see the AJAX spinning icon (Updating cart…), and if I close the modal window and go to checkout there’s nothing in the cart.

I see this error in the console after clicking Add to Cart:

Fatal error: Call to a member function remove_item() on a non-object in /home/username/public_html/wp-content/plugins/wp-e-commerce/wpsc-components/theme-engine-v1/helpers/ajax.php on line 137

https://ww.wp.xz.cn/plugins/wp-e-commerce/