validation function seems to be totally ignored on save()

Having a super-difficult time getting basic validation to happen on a model. Here's my very basic controller action for "providers/add":

public function addAction()
    {
        if ($this->request->isPost()) {
            $provider = new Providers();
            if ($provider->save($this->request->getPost())) {
                return $this->response->redirect('');
            } else {
                $this->flashSession->error(implode(', ', $provider->getMessages()));
                return $this->response->redirect('providers/add');
            }
        }
    }

And here is my model's validation function:

public function validation()
    {
        $this->validate(new PresenceOf(['field' => 'last_name']));
        $this->validate(new Email(['field' => 'email', 'allowEmpty' => true]));
        return $this->validationHasFailed() != true;
    }

The validation function seems to be totally ignored, because I never get an error message even when last_name is an empty string. I have even tried echoing some info in the validation function followed by "exit" but nothing happens, so it seems the validation function is never even run. Any ideas?