Exceção no EJB session stateful

-- BEAN
package cart_session;

import javax.ejb.*;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

/* the bean class itself */
public class CartBean implements SessionBean {
  SessionContext sessionContext;
  java.lang.String _cardHolderName;
  java.lang.String _creditCardNumber;
  java.util.Date _expirationDate;
  java.util.List _items = new ArrayList(10);

  public void ejbCreate(String cardHolderName, String creditCardNumber, Date expirationDate) throws CreateException {
    /**@todo Complete this method*/
    this._cardHolderName = cardHolderName;
    this._creditCardNumber = creditCardNumber;
    this._expirationDate = expirationDate;
  }

  public void ejbRemove() {
    /**@todo Complete this method*/
  }
  public void ejbActivate() {
    /**@todo Complete this method*/
  }
  public void ejbPassivate() {
    /**@todo Complete this method*/
  }
  public void setSessionContext(SessionContext sessionContext) {
    this.sessionContext = sessionContext;
  }
  public void addItem(Item item) {
    /**@todo Complete this method*/
    System.out.println("\taddItem(" + item.getTitle() + "): " + this);
    this._items.add(item);
  }

  public void removeItem(Item item) {
    /**@todo Complete this method*/
    System.out.println("\tremoveItem(" + item.getTitle() + "): " + this);
    if (! _items.remove(item)) {
      throw new EJBException("The item " + item.getTitle() + " is not in your cart.");
    }
  }

  public java.util.List getContents() {
    /**@todo Complete this method*/
    System.out.println("\tgetContents(): " + this);
    return _items;
  }

  public float getTotalPrice() {
    /**@todo Complete this method*/
    System.out.println("\tgetTotalPrice(): " + this);
    float totalPrice = 0f;
    for (int i = 0, n = _items.size(); i < n; i++) {
      Item current = (Item) _items.get(i);
      totalPrice += current.getPrice();
    }
    return ( (long) (totalPrice * 100)) / 100f;
  }

  public void purchase() {
    /**@todo Complete this method*/
    System.out.println("\tpurchase(): " + this);
    Date today = new Date();
    if (_expirationDate.before(today)) {
      throw new EJBException("Expiration date: " + _expirationDate);
    }
    System.out.println("\tPurchasing not implemented yet!");
  }
}

--INTERFACE REMOTA
package cart_session;

import javax.ejb.*;
import java.util.*;
import java.rmi.*;


/* the bean?s REMOTE INTERFACE */

public interface Cart extends javax.ejb.EJBObject {

  public void addItem(Item item) throws RemoteException;
  public void removeItem(Item item) throws RemoteException;
  public java.util.List getContents() throws RemoteException;
  public float getTotalPrice() throws RemoteException;
  public void purchase() throws RemoteException;

}

--INTERFACE HOME
package cart_session;

import javax.ejb.*;
import java.util.*;
import java.rmi.*;

/* the bean?s HOME INTERFACE */

public interface CartHome extends javax.ejb.EJBHome {

  public Cart create(String cardHolderName, String creditCardNumber, Date expirationDate) throws CreateException, RemoteException;

}


  --CHAMADA DO SERVLET
     (...)
     Object obj = null;
     Context context=null;
     String JNDIName = "java:comp/env/ejb/CartHome";
     Hashtable env = new Hashtable();
     env.put(Context.URL_PKG_PREFIXES, "oracle.aurora.jndi");
     try {
       // Create Initial Context and look up directory service of OC4J for object
       context= new InitialContext( env );
       obj = context.lookup(jndiName);
       cartHome = (CartHome) PortableRemoteObject.narrow(obj, CartHome.class);
        cartHome.create("TesteNome","99999999-89", new Date());

     }catch (NamingException e) {
       e.printStackTrace();
     }
     (...)

--WEB.XML
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN" "http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">
<web-app>
  <servlet>
    <servlet-name>cartservlet</servlet-name>
    <servlet-class>cart_session.CartServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>cartservlet</servlet-name>
    <url-pattern>/cartservlet</url-pattern>
  </servlet-mapping>
  <ejb-ref>
    <ejb-ref-name>ejb/CartHome</ejb-ref-name>
    <ejb-ref-type>Session</ejb-ref-type>
    <home>cart_session.CartHome</home>
    <remote>cart_session.Cart</remote>
    <!-- <ejb-link>cart_session</ejb-link> -->
  </ejb-ref>