**Write a function called digit_sum that takes a positive integer n as input and returns the sum of all that number’s digits. For example: digit_sum(1234) should return 10 which is 1 + 2 + 3 + 4.**

**Wondering why my code below isn’t working. Please help. (local variable ‘sum’ referenced before assignment on line 5)**

```
sum = 0
def digit_sum(n):
number = str(n)
for digit in number:
sum = sum + int(digit)
return sum
print(digit_sum(1234))
```