Mockito in debug mode with breakpoint results to WrongTypeOfReturnValue

There is this simple snippet.

@Component
public class SomeDependency {
    public Optional<Integer> getSomeInt(String string) {
        return Optional.of(1);
    }
}

@Component
public class SomeService {

    @Autowired
    private SomeDependency someDependency;

    public String someMethod(String string) {
        return String.valueOf(someDependency.getSomeInt(string).get());
    }
}

@RunWith(MockitoJUnitRunner.class)
public class SomeServiceTest {

    @Mock
    private SomeDependency someDependency;

    @InjectMocks
    private SomeService someService;

    @Test
    public void test() {
        when(someDependency.getSomeInt(anyString()))
                .thenReturn(Optional.of(111));

        String value = someService.someMethod("test");

        assertThat(value, is("111"));
    }
}

Now, when I run the test, it goes OK, but when I run it in debug mode while having breakpoint on when...thenReturn... mock and use step over, following error is thrown.

org.mockito.exceptions.misusing.WrongTypeOfReturnValue:
        Optional cannot be returned by toString()
        toString() should return String
        ***
        If you're unsure why you're getting above error read on.
        Due to the nature of the syntax above problem might occur because:
        1. This exception *might* occur in wrongly written multi-threaded tests.
        Please refer to Mockito FAQ on limitations of concurrency testing.
        2. A spy is stubbed using when(spy.foo()).then() syntax. It is safer to stub spies -
        - with doReturn|Throw() family of methods. More in javadocs for Mockito.spy() method.

But, if I use following oneliner instead of the twoliner, it's OK. No error returned.

when(someDependency.getSomeInt(anyString())).thenReturn(Optional.of(111));

So, where is the problem?