This is why I advocate for end to end testing of a whole system's expected behavior to augment a small set of unit tests (the unit tests are for edge cases).

Nobody needs division by zero tests if there are already guards in place so that can't happen, but it's quite helpful to have a "A goes in, B should come out" view from a client/user perspective. As long as behavior appears correct to the client and is not exploitable you're good to go.