existsBy projection throws error

[micw]

Example: Inmy UserRepository I have a projection that checks if an admin exists:

boolean existsByAdminIsTrue();

This results in exception: org.springframework.aop.AopInvocationException: Null return value from advice does not match primitive return type for: public abstract boolean
If an admin exists, I get a class cast exception.

Querying for a single admin works:
User findOneByAdminIsTrue();