In Practice makes Perfect 4 .digit_sum I get this error message:

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

And the funny thing is that I get the result 11 when I put 434 in the function.

Here is my code, I don't have any syntax errors in it .

```
def digit_sum(n):
x=str(n)
k=[]
z=0
for i in range(len(x)):
k.append(x[i])
for j in k:
z+=int(j)
print z
digit_sum(434)
```