It depends on what language you are using and what things you are testing. If you use a language like Python, having tests that pass things that aren't the typ

It depends on what language you are using and what things you are testing. If you use a language like Python, having tests that pass things that aren't the type you are expecting are useful. Likewise passing nulls to a nullable parameter, or different objects to an Object/Any type (or even a different implementation/type to a supertype parameter). Those tests are part of the unit test repertoire, but unit tests should cover both expected cases/behaviour and unexpected cases.