How to load initial data with playframework2.1 and ebean for single table inheritance?

Question

I get this error:

" NullPointerException: null No source available, here is the exception stack trace:

->java.lang.NullPointerException:

 com.avaje.ebean.Ebean.save(Ebean.java:585)" 

after clicking on "Apply this script now" on the initial page that states "Database 'default' needs evolution!".

I have single table inheritance (the only one supported by ebean), where Usuario is the parent table and Operador is child.

I have the next models:

@Entity
@Table(name="usuario")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "DTYPE", discriminatorType = DiscriminatorType.STRING)
@DiscriminatorValue("usuario")
public class Usuario {
    @Id
    public Long id;

    @Constraints.Required
    public String username;

    @Constraints.Required
    public String password;

    @Constraints.Required
    public String nombre;

    @Constraints.Required
    public String apellido;

    @Constraints.Required
    @Constraints.Email
    public String email;
}

@Entity
@DiscriminatorValue("operador")
public class Operador extends Usuario {
    @Formats.DateTime(pattern = "dd/mm/yyyy hh:mm:ss")
    public Date ultimaSesion;
}

This is the table definitions for Usuario (defined in /conf/evolutions/defaults/1.sql):

# --- !Ups
create table usuario (
  id serial,
  DTYPE varchar(10) not null,
  nombre varchar(30) not null,
  apellido varchar(35) not null,
  username varchar(30) not null,
  password varchar(255) not null,
  email varchar(255) not null,
  ultima_sesion timestamp,
  constraint pk_usuario primary key (id)
);

As you can see, table Usuario has DTYPE and ultima_sesion (which will be inherited by Operador class)

And this is initial-data.yml:

usuario:
    - !!models.Usuario
        username: usuario1
        password: usuario1pass
        nombre: NombreUsuario1
        apellido: ApellidoUsuario1
        email: email@usuario1.com

    - !!models.Operador
        username: usuario2
        password: usuario2pass
        nombre: NombreUsuario2
        apellido: ApellidoUsuario2
        email: email@usuario2.com
        ultimaSesion: 2013-06-15T17:18:23.18-06:00

    - !!models.Usuario
        username: usuario2
        password: usuario2pass
        nombre: NombreUsuario3
        apellido: ApellidoUsuario2
        email: email@usuario3.com

I load the initial-data using the Global class stored in /app as follows:

public class Global extends GlobalSettings {
    public void onStart(Application app) {
        InitialData.insert(app);
    }

    static class InitialData {
        public static void insert(Application app) {
            if(Ebean.find(Usuario.class).findRowCount() == 0) {
                Map<String, List<Object>> usuario = (Map<String, List<Object>>) Yaml.load("initial-data.yml");
                Ebean.save(usuario.get("usuario"));
            }
        }

    }
}

I can't see where the mistake could be. Maybe it is the way I'm trying to instantiate Operador on initial-data.yml? I suppose it is ok to do !!models.Operador since it is a subclass of Usuario.

Thanks for any help you can provide.