Laravel Validation Extend Not Firing

I don't think that my validator extentions are working, but I can't fathom why.

Service Provider.

class AppServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        Validator::extend('fail', function ($attribute, $value, $parameters, $validator) {
            return false;
        });
    }

    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {
    }

}

I know the Service Provider's boot is firing because if I dd(); inside the boot method I get output. If I add a dd(); to the extend closure function, I do not get any output.

Request

class SaveOrder extends FormRequest
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        $rules = [
            'customer_id' => 'in:' . Customer::getImplodedCurrentTeamKeys(),
            'pin' => 'fail'
        ];

        return $rules;
    }
}

I know the request is validating correctly because if I change the rule to 'pin' => 'required' and don't give in put I get a fail.

Why is my custom validation rule not working?

via Samuel Hawksby-Robinson