My code is generating the correct output, but I am receiving the following error code:

list_function([5, 4]) returned 7 instead of [5, 7]

I don't know what this means or how to correct this error.

Thank you for your help.

```
def list_function(x):
x[1] = x[1] + 3
return x[1]
n = [3, 5, 7]
list_function(n)
print n
```