JMSSerializer and FOSRestBundle - annotations do not work. "Does not exist"

I try to use ExclusionPolicy, but I keep getting the error "Annotations do not exist or cannot be loaded automatically."

Here is the exact error:

[Semantic error] The annotations "@JMS \ SerializerBundle \ Annotation \ ExclusionPolicy" in the class Acme \ DemoBundle \ Entity \ Attributes does not exist or cannot be automatically loaded.

My code is as follows:

namespace Acme\DemoBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints;
use JMS\SerializerBundle\Annotation\ExclusionPolicy;
use JMS\SerializerBundle\Annotation\Expose;

/**
 * Acme\DemoBundle\Entity\Attributes
 *
 * @ORM\Table()
 * @ORM\Entity(repositoryClass="Acme\DemoBundle\Entity\AttributesRepository")
 * 
 * @ExclusionPolicy("all")
 */
class Attributes
{
   ...
}