Writing JUnit tests for restTemplate.delete with Observable throwing OnErrorNotImplementedException

Question

I have two methods in my Spring Boot(version 1.3.3) project as follows;

public Observable<Void> methodA() {
  return this.methodB().map(s -> {
    methodC.subscribe( aVoid ->
      // some code
    );
    // some code
  });
}

private Observable<Void> methodC() {
  return Observable<~>create(sub -> {
    restTemplate.delete(//some code);
    sub.onNext(null);
    sub.onCompleted();
  }).doOnNext(//some code)
  .doOnError(//some code);
}

I am using rxjava version 1.3.8. I am trying to write Junit tests for methodC where restTemplate.delete(//some code); throws a HttpClientErrorException. For that, mocked the restTemplate.delete(//some code); as follows;

@Test
public void test() {
  // some code
  Mockito.doThrow(new HttpClientErrorException(HttpStatus.NOT_FOUND)).when(restTemplate)
    .delete(//some code);
  // some code

  final Observable<Void> result = methodA();

  final TestSubscriber<Void> subscriber = new TestSubscriber();
  result.subscribe(subscriber);
  subscriber.assertNotCompleted();
  subscriber.assertError(HttpClientErrorException.class);
}

The mock is working as expected but it throws rx.exceptions.OnErrorNotImplementedException: 404 NOT_FOUND and fails the test. but when I run the code, it is working as expected.

What am I missing here? How can I mock restTemplate.delete(// some code); to throw HttpClientErrorException and assert it?

Thanks in advance!