#### §4.3.(e) Fragile callin binding

If a role method returns void, but the bound base method declares a non-void result, this is reported as a fragile callin binding: The result can still be provided by the base call, but omitting the base call may cause problems depending on the return type:

- For reference return types `null` will be returned in this case.
- In the case of primitive return types this will cause a `ResultNotProvidedException` at run-time.

It is an error if a callin method involved in a fragile callin binding has definitely no base call.