Internal Collection of a PersistentCollection is an Entity

$reflField->getValue($entity) ?: new ArrayCollection

Fatal error: Call to undefined method Entity\User::clear() in \library\Doctrine\ORM\PersistentCollection.php on line 206

class Organization {
    /**
     * @OneToMany(targetEntity="User", mappedBy="organization", cascade={"persist", "remove"})
     */
    private $users;    

    public function __constructor() {
      $this->users = new ArrayCollection();
   }
}

class User {
    /**
     * @ManyToOne(targetEntity="Organization", inversedBy="users")
     * @JoinColumn(name="organization_id", referencedColumnName="id")
     */	
 	private $organization; 	
}