Mongo ODM @Collection Annotation Usage

Question

I am unsure on how to use the @Collection annotation in Mongo. As you know, I am trying to save a simple array in a Mongo document.

So in my document:

 /**
     * @MongoDB\Collection
     */
    protected $counties = array();



/**
     * Set counties
     *
     * @param collection $counties
     * @return self
     */
    public function setCounties($counties)
    {
        $this->counties = $counties;
        return $this;
    }

    /**
     * Get counties
     *
     * @return collection $counties
     */
    public function getCounties()
    {
        return $this->counties;
    }

Now when I go to save something for example:

$county = "test entry"
    $obj = new Obj();
    $obj->setCounties(array($county));

It now gives me warning "expected collection, got array".

How come?