Java EE (Java Enterprise Edition) General Discussion

ejb_3.0.1
jse 1.6.0_14

error:
java.lang.ClassCastException: app.security.User cannot be cast to app.security.User

code:
EntityManager eman = JpaEntityManagerFactory.getEntityManager();
User user = null;
try {
user = (User)((eman.createNamedQuery("User.findById").setParameter("param", Id)).getSingleResult()); <<<<<<<< exception

NamedQuery in User object:
@NamedQuery(name = "User.findById", query = "select o from User o where o.Id= :param")


Application started throwning errors like this wherever I've been using queries. Any direction as to where I may have gone wrong?