Save generates empty documents when using getter/setters and alt source

I am trying to use a Collection model to create a new record in Mongo. Fetches work fine. I have defined getters/setters and as well an alternative colletion name to be used (getSource()). However upon save() I get no errors but a blank document save for the _id being set.

Could someone point me to the path of enlightenment just in case I have this all wrong or could this be a bug?

TIA

        namespace Apps\Document\Model;
        use \Phalcon\Mvc\Collection;
        class Client extends Collection
        {
            protected $client_id;
            protected $client_secret;

            public function getSource()
            {
                return "app_clients";
            }

            public function setClientId($client_id)
            {       
                $this->client_id = $client_id;
            }   
            public function getClient_Id()
            {
                return $this->client_id;
            }
            public function setClientSecret($client_secret)
            {
                $this->client_secret = $client_secret;
            }
            public function getClientSecret()
            {
                return $this->client_secret;
            }
        }

        $AppDoc = new \Apps\Document\Model\Client();
        $AppDoc->setClientId('some id');
        $AppDoc->setClientSecret('shhhh');
        $AppDoc->save();

This results in the following document

{
    "_id" : ObjectId("539ab922279871b7238b4568")
}