Hi,
Basically, it is not possible to go to the checkout with a cart with free products in it. Plus, we didn't thought that someone would like to give coupons of the amount of the order as the goal of a shop is to make money on the goods on sale, so we didn't add any check on that.
That's something we would have to work on.
Do you have that case for all the orders, or is it just sometimes that you want to do that ? If your customers always place orders without having to pay, you could remove the payment view from the checkout workflow in the configuration. That way the payments won't be collected for the orders made on your website.