Can anyone please explain me why my code is not working? I get an error: **Your function fails on digit_sum(434). It returns None when it should return 11.**

What is wrong? It also doesn't print the sum of 123 when I input in the function

```
def digit_sum(n):
total=0
for i in range(len(str(n))-1):
lastnum = n%10
total = total + lastnum
n = n/10
print digit_sum(123)
```