Javascript error on decimal quantities

I’ve got a WC site running the All in One Product Quantity for WooCommerce plugin. The site sells some items by weight (kg), and so it’s possible to order 1.5kg of an item, and that all seems to work fine if the customer places the order…

Until an order is edited. If you then go to undertake an action (eg. resend order confirmation email), the form validation fails with a Javascript error:

An invalid form control with name=’order_item_qty[8109]’ is not focusable. <input type=?”number” step=?”1″ min=?”0″ autocomplete=?”off” name=?”order_item_qty[8109]?” placeholder=?”0″ value=?”1.5″ data-qty=?”1.5″ size=?”4″ class=?”quantity”>?

Desperate for any ideas on how to resolve this. It’s only an issue once the order has been edited.