FluentValidation - inner collection not validated

Question

Have problem similar to FluentValidation collection properties not validated, unfortunately their fix does not work. Any ideas why underlying OrderValidator is not being involved?

validator.Validate(client, ruleSet: "Production");

public class ClientValidator : AbstractValidator<Client>
{
    public ClientValidator()
    {
        RuleSet("Production", () =>
        {
            RuleFor(client => client.Orders)
                .NotEmpty();

            RuleFor(client => client.Orders)
                .SetCollectionValidator(new OrderValidator());
        });
    }
}

public class OrderValidator : AbstractValidator<Order>
{
    public OrderValidator()
    {
        RuleSet("Production", () =>
        {
            RuleFor(x => x.Items)
                .NotNull();
        });
    }
}

Answer 1

Ok, Items.NotEmpty() should have been used instead of NotNull().