Laravel throw exception when setting a property that doesnt exist

Model.php

<?php

namespace App;

use Exception;
use Illuminate\Database\Eloquent\Model as Eloquent;

class Model extends Eloquent
{
    protected $default = ['id'];
    protected $defined = ['firstname', 'lastname'];

    public function setAttribute($key, $value)
    {
        $defined = array_merge($this->$defined, $this->default);

        if (! in_array($key, $defined)) {
            throw new Exception("Attribute '{$key}' has not been defined.");
        }

        return parent::setAttribute($key, $value);
    }
}