super(factory + " returns null and cannot be used to test instance methods.");
    }
  }

  private static final class SerializableDummyProxy extends DummyProxy implements Serializable {

    private final transient ClassSanityTester tester;

    SerializableDummyProxy(ClassSanityTester tester) {
      this.tester = tester;
    }

    @Override
    <R> R dummyReturnValue(TypeToken<R> returnType) {