Oops try again. Does your digit_sum function take exactly one argument (a positive integer)? Your code threw a "string indices must be integers, not str" error.

What's the problem here?

```
def digit_sum(n):
lst = []
a = str(n)
lst.append(a)
for i in lst:
a[i] = int(a[i])
return sum(lst)
```