We don't have coverage on this line, can you add a test for it?

Alternatively, we could just not do this form of validation in the manager. This is my opinion, and I'm sure there are those who would disagree with me, but I think it's OK to rely on the docblock to communicate to the caller that they need to pass you a string here. If they don't do that, it's not your fault if you don't check to make sure it's a string. It's certainly friendly to do so, but since Python is a dynamic language I don't believe it's expected for you to do this kind of checking. Again, this is an open-handed issue, so do as you please, but if you choose to keep these checks in here I'd want to make sure we hit coverage on it too.