marshaling java objects instantiated from classes which implement interfaces with JAXB

How can we marshal objects into XML files using JAXB when we use interfaces for our classes? I have the following simple classes:

public interface IBook {

    public abstract String getName();

    public abstract void setName(String name);

}
@XmlRootElement
public class Book implements IBook {

    private String name;

    @Override
    @XmlElement(name ="BookTitle")
    public String getName() {
        return name;
    }

    @Override
    public void setName(String name) {
        this.name = name;
    }
}


@XmlRootElement
public class BookStore {

    @XmlElement(name ="BookStoreName")
    public String name;

    @XmlElementWrapper(name ="bookList")
    @XmlAnyElement
    public HashSet<IBook> books= new HashSet<IBook>();

    }

and when I try to marshal an object from BookStore into an XML file, I get the following error:

[com.sun.istack.internal.SAXException2: Weder class de.uni_paderborn.books.Book noch eine der zugehörigen Superklassen ist diesem Kontext bekannt.

javax.xml.bind.JAXBException: Weder class de.uni_paderborn.books.Book noch eine der zugehörigen Superklassen ist diesem Kontext bekannt.]

Sorry for the German error message, but my OS is German. This means that neither the class Book nor one of its superclasses is known in this context! Why do I get such an error?