When I use the "Try it" feature, my code produces the right answer without an issue. However, when I click "Run and check", the same call doesn't work. I've provided a screenshot to better communicate what I mean.

From: http://www.checkio.org/mission/restricted-sum/solve/

HTTP_USER_AGENT:

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.27 Safari/537.36