EntityType & # 39; uri # 39;没有定义键

我有一个问题EF 4.1

这里是我的实体和上下文:

 public class PasmISOContext : DbContext
 {
     public DbSet<Avatar> Avatars { get; set; }
     public DbSet<User> Users { get; set; }
 }
namespace PasmISO.Domain
{
    public class User
    {
        [Key]
        public string Login { get; set; }
        public string Email { get; set; }
        public string Password { get; set; }
        public Avatar Avatar { get; set; }
    }
}
namespace PasmISO.Domain
{
    public class Avatar
    {
        [Key]
        public int Id { get; set; }
        [Required]
        public Uri Link { get; set; }
    }
}

在mvc控制器中，我有

 PasmISOContext db=new PasmISOContext();
 if (db.Avatars.Count()==0)
 {
     db.Avatars.Add(new Avatar() { Link = new Uri("http://www.google.com/somelinktojpg") });
 }

当我运行我的代码，我得到异常:

One or more validation errors were detected during model generation:
    System.Data.Edm.EdmEntityType: : EntityType 'Uri' has no key defined. Define the key for this EntityType.
    System.Data.Edm.EdmEntitySet: EntityType: EntitySet �Uris� is based on type �Uri� that has no keys defined.

有什么办法解决这个问题吗?