I am stuck on the chapter 8 (loops) on section 4 (digit_sum) in Python:

https://www.codecademy.com/courses/python-intermediate-en-rCQKw/0/4?curriculum_id=4f89dab3d788890003000096

My code is supposed to add up the digits of a number, however this message pops up:

Your function fails on digit_sum(434). It returns None when it should return 11.

I'm expecting an 11 to be returned, but it is return none in stead.

```
def digit_sum(n):
digits = []
index = len(str(n))
sUm = 0
for number in range(0, len(str(n))):
digits.append(str(n)[number])
for digit in digits:
sUm += int(digit)
print sUm
print digit_sum(434)
```