whats wrong with my function

# E.g. word_count("I am that I am") gets back a dictionary like:
# {'i': 2, 'am': 2, 'that': 1}
# Lowercase the string to make it easier.
# Using .split() on the sentence will give you a list of words.
# In a for loop of that list, you'll have a word that you can
# check for inclusion in the dict (with "if word in dict"-style syntax).
# Or add it to the dict with something like word_dict[word] = 1.

def word_count(sentence):
    word_dict = dict()
    count_dict = dict()
    count = 0
    sentence_list = sentence.split()
    for word in sentence_list:
        word_dict[word] = count
        if word in word_dict:
            word_dict[word] = count + 1
            count_dict.update(word_dict)
    return word_dict