Actually, to be exact, it does allow you to apply the code the first time you try, but when you click through to place the order you get:

“Your order could not be created, the applied voucher is no longer valid and was removed. Please check the basket and click the “Send Order” button again.”

Then, if you try again you get the above “We’re sorry, you cannot use this voucher.” message when you try to apply the code.