"The Most Wanted Letter" doesn't work properly

I would like to give some feedback about ...

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

HTTP_USER_AGENT:

Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.86 Safari/537.36

This code does'n work properly on site, but it should, you can check it locally

import string
dict_alphabet = dict.fromkeys(string.ascii_lowercase, 0)

def checkio(text):
    list_result = []
    for i in text:
        if i.lower() in dict_alphabet:
            dict_alphabet[i.lower()] +=1
    max_val = max(dict_alphabet.values())
    for i in dict_alphabet:
        if dict_alphabet[i] == max_val:
            list_result.append(i)
    list_result.sort()
    return list_result[0]

print checkio("One")

It returns "e", but fails in checkio