Probably a bug

I would like to give some feedback about the test #2

From: https://py.checkio.org/mission/restricted-sum/solve/

HTTP_USER_AGENT:

Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36

My code:

def checkio(data):
    global s
    a = data.pop()
    s = s + a
    if data != []:
        return checkio(data)
    else:
        return s

Second test returns 18, but in console is the right answer, 12.