Stock by Attributes-Allows for checkout when insufficent quantity

Running stock by attributes. I have a test product set up for a shirt with attributes of sizes:
Small, Medium, Large, XL
I've populated the sizes for medium and large and have left xl and small with zero.
When I enter size small or xl to purchase, it allsow the sale to go thru, when it should not.
The only way a sale will be stopped is when the quantity entered for the purchase is in excess of the total quantity of all sizes for this product, not if it is in excess for just one size.
I have a clothing store. I have it down for maintenance.
I am running Zen Cart ver 1.3.9.f where stock by attributes is preinstalled.
I have Allow Checkout set to False.

Any help or advice would be greatly appreciated.

Thanks
:frusty:

I have the same problem and concluded this needs some coding;

A possible solution is to remove the attribute when stock reaches 0 from the drop down (in my case) so the customer cannot select it in the first place.

This is a big customer experience issue for me and am happy to fund a fix to a sensible amount, can anyone help?

Cheers,

Frenc,hy.