Cannot pass null model to Validate. (Parameter 'instanceToValidate')

I need to validate PinCode send from UI, this value need to check with database if not null. I have this code to validate using Fluent Validations:

public class PinCodeValidator : AbstractValidator<string?>
{
    public PinCodeValidator()
    {
        When(r => !string.IsNullOrEmpty(r), () =>
        {
            RuleFor(x => x)
               .Must((x) => { return isPinCodeExists(x).Result; })
               .WithMessage("PinCode details not found !.");
        });
    }

    private Task<bool> isPinCodeExists(string isin)
    {
        // to check with database 
    }
}

I have also added DI to services in ASP.NET Core 8 Web API project as shown here:

services.AddTransient<IValidator<string?>, PinCodeValidator>();            

In the controller action method, I am validating using this code:

[HttpGet]        
public async Task<IActionResult> GetByPinCode([FromQuery] string? pincode)
{
    // validate
    this._isinValidator.ValidateAndThrowMISVAppException(pincode);           
}

Something is wrong with this code, I get an exception

Cannot pass null model to Validate. (Parameter 'instanceToValidate')

Any idea what I am doing wrong here?