bug on accepted code?

I would like to give some feedback about ...

From: http://www.checkio.org/mission/most-wanted-letter/solve/

HTTP_USER_AGENT:

Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.73 Safari/537.36

I am using the code below

from collections import Counter

text = 'one' arr = list(text)

arr=filter(lambda x:x.islower(),arr)

print Counter(arr).most_common(1)[0][0]

which it will yield the correct result 'e', but on the editor on checkio it will show the result of my code is 'n'