Write unit tests for a function

Write unit tests for the function below using {{test_framework}}.

Cover:
- The normal case
- Edge cases (empty input, boundaries, unusual types)
- Error cases
Name each test so it reads as a sentence describing the behavior.
If the function is hard to test (hidden dependencies, time, randomness), say what to change and show the smallest refactor.

```{{language}}
{{code}}
```