GAE & Siena - abstract classes and instantiation exceptions

I am building my first GAE app using the Play! Framework and I am having issues with Siena and abstract classes.

A feature within the app is to allow a user to upload a post on which other users can comment. However, I am running into problems when I try to create multiple post types which inherit from an abstract Post class.

The Post class and a respective Picture subclass are defined as follows:

Post

public abstract class Post extends Model {

    //***EDIT*** - added default constructor
    public Post(){

    }

    @Id(Generator.AUTO_INCREMENT)
    public long id;

    @Filter("post")
    public Query<Comment> comments;

    public static Query<Post> all() {
        return Model.all(Post.class);
    }

    public static Post findById(long id) {
        return all().filter("id", id).get();
    }

    public Comment addComment( User user, String s_comment ){
        Comment comment = new Comment( this, s_comment );
        return comment;
    }

    public List<Comment> comments() {
        return comments.order().fetch();
    }
}

Picture

public class Picture extends Post {

    //***EDIT*** - added default constructor
    public Post_Pic(){

    }

    public String path;
    public String description;

}

And the respective Comment class:

Comment

public class Comment extends Model {

    @Id(Generator.AUTO_INCREMENT)
    public long id;

    @Index("post_index")
    public Post post;

    public String comment;

    public static Query<Comment> all() {
        return Model.all(Comment.class);
    }

    public static Comment findById(long id) {
        return all().filter("id", id).get();
    }

    public Comment( Post post, String comment ){
        this.post    = post;
        this.comment = comment;
    }
}

The problem that I am having is that when I try and get comments for a Picture object, an InstantiationException is thrown.

Does Siena (or Java) try to create a new instance of Post when retrieving a Comment and as a result throw an InstantiationException? If so, can anyone point me in the right direction for achieving what I am trying to do?

*** Edit ***

I have added default constructors to my models but this has unfortunately had no effect.

The test code which is throwing the error is as follows:

@Test
public void commentPost_Pic(){
    User bob = new User( fullname, email, password );
    bob.insert();
    bob = User.connect( email, password );
    assertNotNull( bob );

    Post_Pic post_pic = new Post_Pic();
    post_pic.insert();
    Comment comment = post_pic.addComment( bob, testComment );
    comment.insert();

    List<Comment> comments = post_pic.comments();       //**** <- Error gets thrown here
    assertEquals( 1, comments.size() );
}

The InstantiationException is thrown at the line List<Comment> comments = post_pic.comments();.

Try to create a default constructor in your models... Siena uses reflection to create classes but it doesn't try to use specific constructors! Tell me if it works better!