MockWebServer catch errors

I'm using RxJava, Retrofit and MockWebServer I'm doing unit tests on my services.

My restAdapter have a custom error handler returning a custom throwable depending on the error code :

RestAdapter restAdapter = new RestAdapter.Builder().setErrorHandler(new RetrofitErrorHandler(context))

I tried to use a TestSubscriber and to subscribe like this:

    TestSubscriber<X> testSubscriber = new TestSubscriber<>();
    Observable<X> observable = mService.myCall(null, email);

    observable.subscribe(testSubscriber);

but the .getOnErrorEvents() returns 0 event.

What should I do ?