Potential Bug with Most Wanted Letter

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; WOW64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31

My problem: Your page keeps telling me I failed when it runs the "try it" Lorem ipsum text through my code. All other checks pass just fine. The page is pulling a bunch of 0's into the text which I don't see, and it won't let me submit my code and move on (even though all other assertions are true). I can provide a screenshot if necessary.

My Code:

def checkio(text):
    output = text.lower() #make it lowercase, caps dont matter

    alreadycounted = []
    highest = 0
    counter = []

    for o in output:
        if o not in alreadycounted and o.isalnum() == True:

            if output.count(o) > highest:
                highest = output.count(o)
                counter = []
                counter.append(o)
            elif output.count(o) == highest:
                counter.append(o)

            alreadycounted.append(o)

    counter.sort()
    #print("Your word is: " + str(text))
    print("Highest letter: " + str(counter) + " with " + str(highest) + " occurrences")

    if len(counter) > 0:
        return counter[0]
    else:
        return False

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.")