EF no clearcache causes error on multiple updates

            using (var ctx = new EntityContext())
            {
                var newCustomer = new Customer
                {
                    Description = "test",
                    Name = "SOTest",
                    IsActive = true
                };

                ctx.Customers.Add(newCustomer);
                ctx.SaveChanges();

                var customerID = newCustomer.CustomerID;
                var customer = ctx.Customers
                    .AsNoTracking()
                    .Where(x => x.CustomerID == customerID)
                    .Select(x => x)
                    .FirstOrDefault();
                customer.Description = "Some nasty ass description";

                ctx.ClearCache();
                ctx.Entry(customer).State = EntityState.Modified;
                ctx.SaveChanges();

                var anotherCustomer = ctx.Customers
                    .AsNoTracking()
                    .Where(x => x.CustomerID == customerID)
                    .Select(x => x)
                    .FirstOrDefault();
                anotherCustomer.Name = "Trying once more";

                //ctx.ClearCache();
                ctx.Entry(anotherCustomer).State = EntityState.Modified;
                ctx.SaveChanges();
            }

    public class Customer
    {
        public int CustomerID { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
        public Boolean IsActive { get; set; }
    }

    public class EntityContext : DbContext
    {
        public EntityContext() : base("DefaultConnectionString")
        {
            InitContext();
        }

        void InitContext()
        {
            /// Should be enabled by default, if optimization is needed then turn it off.
            Configuration.AutoDetectChangesEnabled = true;
            /// We do not use proxies for lazy loading nor change-tracking.
            Configuration.ProxyCreationEnabled = false;
            /// Should always be false!
            Configuration.LazyLoadingEnabled = false;
            ///We set this to true to avoid unnecessary check in SQL
            ///(if sending null parameter to query, SQL will check if column is null and parameter is null and we do not use this kind of filtering)
            Configuration.UseDatabaseNullSemantics = true;
            //Default timeout for connection
            Database.CommandTimeout = 60;
        }
        public void ClearCache()
        {
            Configuration.AutoDetectChangesEnabled = false;
            var trackedItems = ChangeTracker.Entries().ToList();
            foreach (var item in trackedItems)
            {
                Entry(item.Entity).State = System.Data.Entity.EntityState.Detached;
            }
            Configuration.AutoDetectChangesEnabled = true;
        }

        public DbSet<Customer> Customers { get; set; }
    }