Fluent Validation with SetValidator should error when it doesn't.

gistfile1.cs

using FluentValidation;
using FluentValidation.TestHelper;
using Xunit;

namespace FVSetValidatorTest
{
    #region Models

    public class UnitOfMeasure
    {
        public string Type { get; set; }
        public decimal Value { get; set; }
    }

    public class LandDetails
    {
        public UnitOfMeasure Area { get; set; }
        public UnitOfMeasure Frontage { get; set; }
    }

    #endregion

    #region Validator's

    public class UnitOfMeasureValidator : AbstractValidator<UnitOfMeasure>
    {
        public UnitOfMeasureValidator()
        {
            RuleFor(unit => unit.Value).GreaterThanOrEqualTo(0);

            RuleFor(unit => unit.Type).NotNull()
                .When(unit => unit.Value > 0)
                .WithMessage("If a unit of measure's 'Value' is provided, then a 'Type' also needs to be provided.");
        }
    }

    public class LandValidator : AbstractValidator<LandDetails>
    {
        public LandValidator()
        {
            RuleFor(land => land.Area).SetValidator(new UnitOfMeasureValidator());
            RuleFor(land => land.Frontage).SetValidator(new UnitOfMeasureValidator());
        }
    }

    #endregion

    public class LandValidatorFacts
    {
        [Fact]
        public void GivenANegativeValue_Validate_ShouldHaveAValidationError()
        {
            var landDetails = new LandDetails
            {
                Area = new UnitOfMeasure
                {
                    Value = -111
                }
            };

            var validator = new LandValidator();
            validator.ShouldHaveChildValidator(land => land.Area, typeof(UnitOfMeasureValidator));

            // This line of code SHOULD pass, but it doesn't because the validation doesn't error.
            validator.ShouldHaveValidationErrorFor(land => land.Area, landDetails.Area);

            // This works....
            var results = validator.Validate(landDetails);
            Assert.Equal(1, results.Errors.Count);
        }
    }
}