MarketPress Inventory of 1 Problem

It appears as if a product with an inventory of 1, cannot be purchased.
The "Add to Cart" button appears, but does not function. There's also a message above the button which reads "We're sorry, we only have 1 of this item in stock right now." Basically, it looks like it won't let anyone buy the last of a product.

The problem is not present if the quantity is set to 2+. The problem is also not present if the "Allow this product to be purchased even if it's out of stock" checkbox is checked. However, I need to be able to sell unique items with a quantity of 1, after which they would need to be visible as "Out of Stock".

Any suggestions?