Fluent Validation 3.2 not working

hi,
     I am facing some problem in validating the expiration date in fluent validtion 3.2 using nop commerce 2.40. where  i am using greater than or equalsto it is not taking the validation for it. i am checking the validation for the month should not be less than the current month and i am using dropdownlist to select the month .

here is my code:

string month = DateTime.Now.Month.ToString();

RuleFor(x => x.ExpireMonth).GreaterThanOrEqualTo(month).WithMessage("invalid expiry date");

it is not executing this validation even if i select less than the current month..

help would be appreciated...

It doesn't work with this rule:

RuleFor(x=>x.CategoryId)
                .GreaterThan(0)
                .WithMessage(localizationService.GetResource("Admin..."));

Please help