Sequence contains no matching element

 public partial class User
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int UserId { get; set; }

        [Required]
        [StringLength(20)]
        public string UserName { get; set; }

        [Required]
        [StringLength(50)]
        public string UserPassword { get; set; }

        public DateTime? LastModified { get; set; }

        public bool? Inactive { get; set; }

        [StringLength(50)]
        public string Firstname { get; set; }

        [StringLength(50)]
        public string Lastname { get; set; }

        [StringLength(30)]
        public string Title { get; set; }

        [StringLength(3)]
        public string Initial { get; set; }

        [StringLength(100)]
        public string EMail { get; set; }

        public DateTime? UserLastLogin { get; set; }

        public int? FailedLoginAttempts { get; set; }

        
        [StringLength(1)]
        public string PasswordChangeatLogin { get; set; }

        public bool? IsAdmin { get; set; }
    }
}

public ActionResult Login(Models.User user)
        {
            //        var errors = ModelState
            //.Where(x => x.Value.Errors.Count > 0)
            //.Select(x => new { x.Key, x.Value.Errors })
            //.ToArray();
            if (ModelState.IsValid)
            {
                if (IsValid(user.UserName, user.UserPassword))
                {
                    FormsAuthentication.SetAuthCookie(user.UserName, true);
                    return RedirectToAction("Default", "Home");
                }
                else
                {
                    ModelState.AddModelError("", "Login data is incorrect!");
                }
            }
            return View(user);
        }

 private bool IsValid(string username, string password)
        {

            bool IsValid = false;

            using (var db = new PowerERP.Models.PowerModel())
            {
                var user = db.Users.FirstOrDefault(u => u.UserName == username);
                if (user != null)
                {
                    var encryptedPassword = CryptorEngine.Encrypt(password.Trim(), true);
                    if (user.UserPassword == encryptedPassword)
                    {
                        IsValid = true;
                    }
                }


            }
            return IsValid;

        }