I have 2 Model

AccountEntity

[Table(Name = "Account")]
    public class AccountEntity
    {
        [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert)]
        public int id { get; set; }
        [Column(CanBeNull = false, Name = "email")]
        public string email { get; set; }
        [Column(CanBeNull = false, Name = "pwd")]
        public string pwd { get; set; }
        [Column(CanBeNull = false, Name = "row_guid")]
        public Guid guid { get; set; }

        private EntitySet<DetailsEntity> details_id { get; set; }
        [Association(Storage = "details_id", OtherKey = "id", ThisKey = "id")]
        public ICollection<DetailsEntity> detailsCollection { get; set; }
    }

DetailsEntity

[Table(Name = "Details")]

public class DetailsEntity
{
    public DetailsEntity(AccountEntity a) {
        this.Account = a;
    }

    [Column(IsPrimaryKey = true, IsDbGenerated = true, DbType = "int")]
    public int id { get; set; }

    private EntityRef<AccountEntity> _account = new EntityRef<AccountEntity>();
    [Association(IsForeignKey = true, Storage = "_account", ThisKey = "id")]
    public AccountEntity Account { get; set; }
}

Main

 using (Database db = new Database())
 {
     AccountEntity a = new AccountEntity();
     a.email = "hahaha";
     a.pwd = "13212312";
     a.guid = Guid.NewGuid();
     db.Account.InsertOnSubmit(a);
     db.SubmitChanges();
 }

that has relationhip AccountEntity <- DetailsEntity (1-n)

when i'm trying to insert a record exception throws NullReferenceException

cause: by EntitySet null

please help me make it insert