RxJava TestSubscriber / TestSingleSubscriber

I have troubles with writing a test for Single operator. For some reason Subscriber is marked as unsubscribed just after creation.

Snippet of code:

public abstract class OneShotUseCase<T> {

    @NonNull
    private CompositeSubscription subscription = new CompositeSubscription();

    public void cancel() {
        subscription.clear();
    }

    public void execute(@NonNull SingleSubscriber<T> subscriber) {
        subscription.add(
                build().subscribe(subscriber)
        );
    }

    @NonNull
    protected abstract Single<T> build();
}

Test for that:

@RunWith(JUnit4.class)
public class OneShotUseCaseTest {

    private OneShotUseCase<String> sut;


    @Before
    public void setUp() {
        MockitoAnnotations.initMocks(this);
        sut = new TestUseCase();
    }

    @Test
    public void shouldBuildUseCase() {
        final TestSingleSubscriber<String> stringTestSingleSubscriber = new TestSingleSubscriber<>();

        sut.execute(stringTestSingleSubscriber);

        stringTestSingleSubscriber.assertValue("Test value");
    }

    @Test
    public void shouldCancelUseCase() {
        final TestSingleSubscriber<String> stringTestSingleSubscriber = new TestSingleSubscriber<>();

        sut.execute(stringTestSingleSubscriber);
        sut.cancel();

        stringTestSingleSubscriber.assertUnsubscribed();
    }


    private static class TestUseCase extends OneShotUseCase<String> {

        @NonNull
        @Override
        protected Single<String> build() {
            return Single.just("Test value");
        }
    }
}

As TestSingleSubscriber I am using following code: http://ift.tt/29xr1JY

My test fails at line stringTestSingleSubscriber.assertUnsubscribed();

I tried to dig little bit, but I found nothing before for such issue.

Is it something wrong with my understanding of RxJava Single?

Worth to mention that very similar code with Observable<T> instead of Single works in test perfectly.

I hope to find some ideas here how to make it working.

Thanks.