Untitled

EF Upgrade from 4.0.0 to 4.3.1 causing strange errors
public class ItemBase
{
    public DateTime Created { get; set; }
    public int CreatedByUserID { get; set; }
    public DateTime LastModified { get; set; }
    public int LastModifiedByUserID { get; set; }

    public User CreatedBy { get; set; }
    public User LastModifiedBy { get; set; }

    public ItemBase()
    {
        CreatedByUserID = -1;
        LastModifiedByUserID = -1;
        CreatedBy = null;
        LastModifiedBy = null;
    }
}


public class User : ItemBase
{
    public int UserID { get; set; }
    public string LoginName { get; set; }
    public string Password { get; set; }
    public string EmailAddress { get; set; }
    public string Firstname { get; set; }
    public string Lastname { get; set; }
    public string DisplayName { get; set; }

    public User() : base()
    {
        UserID = -1;
        LoginName = String.Empty;
        Password = String.Empty;
        EmailAddress = String.Empty;
        Firstname = String.Empty;
        Lastname = String.Empty;
        DisplayName = String.Empty;
    }
}


protected override void OnModelCreating(System.Data.Entity.ModelConfiguration.ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);
    modelBuilder.Entity<User>().Property(u => u.UserID).HasDatabaseGenerationOption(DatabaseGenerationOption.Identity);
}

modelBuilder.Entity<User>().HasRequired(u => u.CreatedBy).WithMany().HasForeignKey(k => k.CreatedByUserID);
modelBuilder.Entity<User>().HasRequired(u => u.LastModifiedBy).WithMany().HasForeignKey(k => k.LastModifiedByUserID);

public class MyContext : DbContext
{
    public MyContext()
     : base("name=MyConnectionStringName")
    {
    }
}