we can improve the existing test-case in the same go. Let's add an assertion on the error message:

```ruby
e = assert_raises(...) do
  ...
end
assert_equal "...", e.message
```

We should also update the name of the remaining test case:

```
# before
test_invalid_belongs_to_dependent_option_nullify_raises_exception

# after
test_belongs_to_invalid_dependent_option_raises_exception
```

This expresses that `nullify` is not a special case anymore but just an invalid option.