Upsell popup opens up even if product is out of stock

Hello and thank you for the work on this plugin.
I do have the option “Up sell” enabled with “Ajax Add To Cart” option activated.
But there’s a problem: even if a product is out of stock, clicking on “Add to cart” button on single product page made the popup appear. It says that a product has been added to cart but obviously it doesn’t because the product is out of stock.
You can see the bug in action here: https://www.frognonsolointimo.it/prodotto/body-fbm-074-con-spallina-e-senza-ferretto/.

Thanks!