What have I missed?

# 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(string):

    my_dict = {}

    string = string.lower()

    my_list = string.split()

    for word in my_list:
        if word in my_list:
            my_dict[word] += 1
        else:
            my_dict[word] = 1


    return my_dict