Error at _context.SaveChanges();

customerscontroller.cs

[HttpPost]
        public ViewResult Save(Customer customer )
        {
            if (customer.Id == 0)
            {
                _context.Customers.Add(customer);
            }
            else
            {
                var customerInDB = _context.Customers.Single(c => c.Id == customer.Id);
                //TryUpdateModel(customer, "", new String[] {"Name", "Email"} );
                //Mapper.Map(customer, customerinDb)
                customerInDB.Name = customer.Name;
                customerInDB.Birthdate = customer.Birthdate;
                customerInDB.IsSubscribedToNewsletter = customer.IsSubscribedToNewsletter;
                customerInDB.MembershipType = customer.MembershipType;
                _context.SaveChanges();
            }
            _context.SaveChanges();

            RedirectToAction("Index", "Customers");
            return View();
        }