RxJava2: onErrorResumeNext not intercepting 400 response code

Question

I have below code for error handling. My issue is that when server sends a 400 error code the onErrorResumeNext is not being called. Is this expected? I thought onErrorResumeNext would be called when server sends a non 200 code(200-300).

public Single adapt(Call c) {
 return ((Single) adapt(c)
         .onErrorResumeNext(new Function() {
         //BELOW NOT BEING EXECUTED ON 400 error from server.
         @Override
         public Object apply(Object throwable) throws Exception {

                 return Single.error(DoSomethingWithException((Throwable) throwable));
                 }
         }).subscribeOn(Schedulers.newThread())
           .observerOn(AndroidSchedulers.mainThread())
           .flatMap( response -> {

          }
          );}