Why getting a 'Bummer'?!

# E.g. word_count("I do not like it Sam I Am") gets back a dictionary like:
# {'i': 2, 'do': 1, 'it': 1, 'sam': 1, 'like': 1, 'not': 1, 'am': 1}
# Lowercase the string to make it easier.

def word_count(string):
    string = string.lower()
    words = string.split(' ')
    unique_words = []
    for word in words:
        if word not in unique_words:
            unique_words.append(word)
    counters = []
    for unique_word in unique_words:
        counter = 0
        while unique_word in words:
            words.pop(words.index(unique_word))
            counter += 1
        counters.append(counter)
    return dict(zip(unique_words, counters))

word_count("I do not like it Sam I Am")