Mocking Java 8 lamba function with Mockito 2

I am using JDBI 3 and have written the following utility function.

public class JdbiHelper {
    // ...

    public <T, DAO> T fetch(final Function<DAO, T> function, Class<DAO> daoClass) {
        return dbi.withHandle(handle -> {
           final DAO dao = handle.attach(daoClass);
           try {
               return function.apply(dao);
           }
           finally {
               handle.close();
           }
        });
    }
}

which I can call in methods like this

public Optional<Account> findByEmailAddress(final String emailAddress) {
    if (!exists(emailAddress))
        return Optional.empty();
    return jdbiHelper.fetch(dao -> ofNullable(dao.selectByEmailAddress(emailAddress)), AccountDAO.class);
}

private boolean exists(final String emailAddress) {
    return jdbiHelper.fetch(dao -> dao.count(emailAddress) > 0, AccountDAO.class);
}

I am trying to write a test for the findByEmailAddress mocking jdbiHelper using Mockito 2 but cannot work out how to mock the dao -> part of the method.

I've tried using jdbiHelper.fetch(any(Function.class), eq(AccountDAO.class)) but as there are two different expectations of what to return it fails trying to cast one or the other.

Passing in a mocked Function causes a NPE as the dao param is null.