Why does this EF insert with IDENTITY_INSERT not work?

This is the query:

using (var db = new AppDbContext())
{
    var item = new IdentityItem {Id = 418, Name = "Abrahadabra" };
    db.IdentityItems.Add(item);
    db.Database.ExecuteSqlCommand("SET IDENTITY_INSERT Test.Items ON;");
    db.SaveChanges();
}

When executed, the Id of the inserted record, on a new table, is still 1.

When I use either the transaction, or TGlatzer's answer, I get the exception:

Explicit value must be specified for identity column in table 'Items' either when IDENTITY_INSERT is set to ON or when a replication user is inserting into a NOT FOR REPLICATION identity column.