Entity Framework Extensions Ignore OnInsert Expression

using (var context = new EntityContext())
{
    var customers = new List<Customer>();
    customers.Add(new Customer() { Name = "Alexander", Description = "Description of Alexander", CreatedDate = DateTime.Now, ModifiedDate = DateTime.Now, IsActive = true });
    customers.Add(new Customer() { Name = "Carson", Description = "Description of Carson", CreatedDate = DateTime.Now, ModifiedDate = DateTime.Now, IsActive = true });
    customers.Add(new Customer() { Name = "Meredith", Description = "Description of Meredith", CreatedDate = DateTime.Now, ModifiedDate = DateTime.Now, IsActive = true });
    customers.Add(new Customer() { Name = "Arturo", Description = "Description of Arturo", CreatedDate = DateTime.Now, ModifiedDate = DateTime.Now, IsActive = true });

    context.BulkInsert(customers, options => 
    {
        options.IgnoreOnInsertExpression = customer => new {customer.CustomerID,  customer.ModifiedDate};
    });
}