Free download not checking out

We are having a sale where we are offering a free MP3 at $0. However, if people
do not buy anything else in the store and try to check out with only the $0
amount for the MP3, they are getting an error message that says “please enter
valid amount”. We need to be able to have people check out with only the free
MP3 (or any other future free product) in their cart. How do I do this?

We use authorize.net as our payment option and our finance guy said it’s nothing
on their end.

This problem has been happening about a week now, so if we could get it resolved
quickly, that would be most appreciated!

Please let me know if you need any other information.