Hello,

in the end I looked for the solution to this exercise but am however still curious to know why exactly my code was given “NoneType object is not subscriptable”, that way I hopefully don’t repeat the same error. So this was my code:

```
def append_sum(lst):
sum1 = lst[-1] + lst[-2]
lst = lst.append(sum1)
sum2 = lst[-1] + lst[-2]
lst = lst.append(sum2)
sum3 = lst[-1] + lst[-2]
lst = lst.append(sum3)
return list
```

Thank you.