issue with str_diff_solution (python)

`
class Solution(object):

def find_diff(self, str1, str2):
if str1 is None or str2 is None:
raise TypeError('str1 or str2 cannot be None')
seen = {}
for char in str1:
if char in seen:
seen[char] += 1
else:
seen[char] = 1
for char in str2:
try:
seen[char] -= 1
except KeyError:
return char
if seen[char] < 0:
return char

for char, count in seen.items():
return char
`

Fails for input
`
Solution().find_diff('bbaabbc', 'aabbbb')
`

Last for loop should be
`
for char, count in seen.items():

if count:
return char
`