Using enum as id

Using JPA, can we define an enum as id of an entity?

I've tried the following:

public enum AssetType {
   ....
}

@Entity
@IdClass(AssetType.class)
public class Adkeys {

   private AssetType type;

   @Id
   @Enumerated(EnumType.STRING)
   @Column(nullable = false)
   public AssetType getType() {
      return type;
   }

}

Using OpenJPA, it complains:

org.apache.openjpa.persistence.ArgumentException: The id class "class aa.AssetType" specified by type "class aa.Adkeys" does not have a public no-args constructor.

So my questions are:

• should we able to use enum as id for an entity on JPA? (i.e. there is a bug in OpenJPA)
• or do I make a mistake somewhere?
• and is there any workaround for such problem?