Using class Counter does not work?

Hello,

I try to solve most-wanted-letter problem with class collections.Counter. This works well if I test manually but when I click Run&Check, I even cannot pass "Hello world!".

It says "your result: "null". What is wrong here?

------

<code>

------

import collections

def checkio(text):
    a = (text.lower().replace(" ",""))
    b = (collections.Counter(a).most_common(2))
    c = str(b)
    if str.isalpha(c[3]) == True:
        print (c[3])

    else:
        print (c[13])


if __name__ == '__main__':
    #These "asserts" using only for self-checking and not necessary for auto-testing
    assert checkio("Hello World!") == "l", "Hello test"
    assert checkio("How do you do?") == "o", "O is most wanted"
    assert checkio("One") == "e", "All letter only once."
    assert checkio("Oops!") == "o", "Don't forget about lower case."
    assert checkio("AAaooo!!!!") == "a", "Only letters."
    assert checkio("abe") == "a", "The First."
    print("Start the long test")
    assert checkio("a" * 9000 + "b" * 1000) == "a", "Long."
    print("The local tests are done.")