Thank you! I actually just removed createQty from the code. not only it had a typo, it was not being used anywhere beside the tests.

And, as some commenter on Disqus said, probably the right thing to do would be returning an Qty option from the function, and treating it properly.

I have no experience with Scheme, but already worked with Clojure - it was actually my "gateway" to functional programming languages, hehe!