How to use spock @SpringSpy with a JpaRepository

I am using spring boot with jpa and spock. I want to assert that certain calls were made to the repo as a functional test, so a Mock won't do (I need to verify that a native query works with certain test data in the DB).

I have tried a @SpringBootTest with:

@SpringSpy
MyJpaRepository repo

interaction {
        1 * repo.someMethod(_) 
}

where MyJpaRepository extends JpaRepository<Foo, Long>, but when I run the test I get:

Cannot create mock for class com.sun.proxy.$Proxynnn because Java mocks cannot mock final classes. If the code under test is written in Groovy, use a Groovy mock.

Other than this error, all my functional tests work, so it's not a config issue.

How can I spy on an @Autowired bean?