JaxB UNMarshalling Problem

@XmlElements({
        @XmlElement(name = "Dog" , type = Dog.class),
        @XmlElement(name = "Cat" , type = Cat.class)
})
private List<Animal> animals;

public static void main(String[] args) throws Exception {
    Zoo zoo = new Zoo();
    zoo.animals = new ArrayList<Animal>();

    Dog doggy = new Dog();
    doggy.setDogProp("Doggy");

    Cat catty = new Cat();
    catty.setCatProp("Catty");

    zoo.animals.add(doggy);
    zoo.animals.add(catty);

    JAXBContext jc = JAXBContext.newInstance(Zoo.class, Dog.class, Cat.class);
    Marshaller marshaller = jc.createMarshaller();

    ByteArrayOutputStream os = new ByteArrayOutputStream();
    marshaller.marshal(zoo, os);

    System.out.println(os.toString());

    Unmarshaller unmarshaller = jc.createUnmarshaller();

    unmarshaller.setEventHandler(new javax.xml.bind.helpers.DefaultValidationEventHandler());

    Zoo unmarshalledZoo = (Zoo) unmarshaller.unmarshal(new ByteArrayInputStream(os.toByteArray()));

    if (unmarshalledZoo.animals == null) {
        System.out.println("animals was null");
    } else if (unmarshalledZoo.animals.size() == 2) {
        System.out.println("it worked");
    } else {
        System.out.println("failed!");
    }
}

public interface Animal {
}

@XmlRootElement
public static class Dog implements Animal {

    private String dogProp;

    public String getDogProp() {
        return dogProp;
    }

    public void setDogProp(String dogProp) {
        this.dogProp = dogProp;
    }
}

@XmlRootElement
public static class Cat implements Animal {

    private String catProp;

    public String getCatProp() {
        return catProp;
    }

    public void setCatProp(String catProp) {
        this.catProp = catProp;
    }
}