JAXB marshaling Map of Lists

I have a map of lists that I need to marshal. I created XML adapater but I keep getting java.util.List is an interface, and JAXB can't handle interfaces. when creating JAXB context. How should I marshal Map of Lists?

This is my code:

@XmlRootElement(name = "myClass")
public class MyClass  {

    @XmlJavaTypeAdapter(MapOfListsAdapter.class)
    protected Map<Integer, List<Condition>> expectedResults;

I have written adapter MapOfListsAdapater for the Map:

public class MapOfListsAdapter extends XmlAdapter<List<MapOfListsEntry>, Map<Integer, List<Condition>>> {

    @Override
    public List<MapOfListsEntry> marshal(Map<Integer, List<Condition>> v) {...}

    @Override
    public Map<Integer, List<Condition>> unmarshal(List<MapOfListsEntry> v) {...}
}

MapOfListEntry has these JAXB annotations:

public class MapOfListsEntry {

    @XmlAttribute
    private Integer key;

    @XmlElementRef
    @XmlElementWrapper
    private List<Condition> value;