Lesson 5 

Since it's possible for rate to be returned as `None`, you'll need to make decisions around that possibility (eg, another `if rate:` situation. Since I don't think `compund_interest()` is prepared to accept a `None` rate, perhaps you should consider processing it inside the `if rate`. If you instantiate total as `None` by just reassigning total a value inside the `if rate:` you can simply return `total` at the end.