Implementation of Undo with SoftDelete

I have a base class all model classes derive from and it implements a SoftDelete behaviour this way:

$this->addBehavior(new \Phalcon\Mvc\Model\Behavior\SoftDelete(
            array(
                'field' => 'deleted_at',
                'value' => date('Y-m-d H:i:s')
            )
        ));

Now I need to implement an Undo feature and I have adde the following method to the Entry model:

/**
     * @access public
     * @static
     * @param number $id ID of the model record
     * @return mixed
     */
    public static function undelete($id)
    {
        $deletedField = 'deleted_at';

        $query = new \Phalcon\Mvc\Model\Query(
            "UPDATE Entry SET $deletedField = NULL WHERE id = :id:",
            \Phalcon\DI\FactoryDefault::getDefault()
        );
        $query->execute(array('id' => $id));
        return true;
    }

The code looks right to me. However, when I call the undelete() method from the controller I get the following exception