Word count python challenge. I can't see what's wrong here.

# 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(a_string):
  a_dict = {}
  asplitstring = asplitstring.split()
  for word in asplitstring:
    if word not in a_dict:
      a_dict[word] = 1
    else:
      a_dict[word] = a_dict[word] + 1
  return a_dict