Catchable Fatal Error: EntityManager could not be converted to string

I wanted to implement some services, and the first thing I did was to define the private $em as the EntityManager as follows:

<?php

namespace Users\UsersBundle\Services;

use Doctrine\ORM\EntityManager;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
use Usuarios\UsersBundle\Entity\User;

/**
 * Class UserManager
 */
class UserManager
{
private $em;

/**
 * @param EntityManager $em
 */
public function __construct(EntityManager $em)
{
    $this->$em = $em;
}
}

An example of a function using the EntityManager inside the very same class:

/**
 * Find all posts for a given author
 *
 * @param User $author
 *
 * @return array
 */
public function findPosts(User $author)
{
    $posts = $this->$em->getRepository('BlogBundle:Post')->findBy(array(
            'author' => $author
        )
    );

    return $posts;
}

However when I call any function like for example the one I show above, I get the following error: Catchable Fatal Error: Object of class Doctrine\ORM\EntityManager could not be converted to string.

I did import the service. What am I missing? Thank you in advance for your support.