外键为空的EF导航属性

我第一次尝试使用导航属性来代替连接。我不能得到的情况下，它通常会完成与左外连接。在下面的示例中，我只得到外键非空的结果。我需要所有的结果。我错过了什么?

public class User
{
    [Key]
    public int UserID {get;set;}
    public String Name {get;set;}
}
public class Shipment
{
    [Key]
    public int ShipmentID {get;set;}
    public int? SignedForByID {get;set;}
    [ForeignKey("SignedForByID")]
    public virtual User SignedForBy{get;set;}
}

导航属性映射:

 Shipment.HasOptional(x=> x.SignedForBy).WithMany()
.HasForeignKey(y=> y.SignedForByID).WillCascadeOnDelete(false);

查询:

var data = (from s in context.Set<Shipment>()
            select new {
                    ShipmentID = s.ShipmentID,
                    SignedForBy = s.SignedForBy
                        });