Code challenge: am I missing a part of the question requirements?

Here is the code challenge: Alright, this one might be a bit challenging but you've been doing great so far, so I'm sure you can manage it. I need you to make a function named word_count. It should accept a single argument which will be a string. The function needs to return a dictionary. The keys in the dictionary will be each of the words in the string, lowercased. The values will be how many times that particular word appears in the string. Check the comments below for an example.

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.

When I run the function locally with these inputs: print(word_count("I do not like it Sam I Am")) print(word_count("Where Did aLl the bEArs go bears bears bears"))

The output is: {'i': 2, 'do': 1, 'not': 1, 'like': 1, 'it': 1, 'sam': 1, 'am': 1} {'where': 1, 'did': 1, 'all': 1, 'the': 1, 'bears': 4, 'go': 1}

So my question is, hard to define, but I am not sure why the code challenge is not getting the expected output.

# 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(unit_string):
    word_list = unit_string.lower().split(' ')
    word_count_dict = {}
    # default to 0
    for word in word_list:
        word_count_dict[word] = 0
    # loop again to add
    for word in word_list:
        word_count_dict[word] += 1
    return word_count_dict