Entity C# add class which inherits from other

Question

I have two classes in my Entity project, User and UserProfile, UserProfile inherit from User.

Some part of my code instantiate new User, than I want to add UserProfile to my sql database, which will be expanding my User class. But I don't know how to write it.

I was trying:

Users user = context.Users.FirstOrDefault(o => o.UserID == UserId);
if (user != null)
{
    UserProfile userProfile= new UserProfile ();
    userProfile.UserID = user.UserID;
    context.Users.AddObject(userProfile);
    context.SaveChanges();
    return "ok";
}

but it's not working. Writing just pure sql it'd be no problem. But I don't know how to write it properly. Thanks for help.