Why is my Symfony2 @UniqueEntity constraint not working at all?

I have the following entity class in my application:

<?php

namespace ...;

// use ...
use Doctrine\ORM\Mapping as ORM;
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;;
// ...

/**
 * @ORM\Table(name="sc_user")
 * @ORM\Entity(repositoryClass="...\UserRepository")
 * @ORM\HasLifecycleCallbacks()
 * @UniqueEntity(fields={"email", "username"})
 */
class User implements UserInterface, \Serializable, EquatableInterface
{
    /**
     * @var integer $id
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string $email
     *
     * @ORM\Column(name="email", type="string", length=255, unique=true)
     *
     * @Assert\NotBlank(groups={"registration"})
     * @Assert\Email(groups={"registration"})
     */
    private $email;

    /**
     * @var string $username
     *
     * @ORM\Column(name="username", type="string", length=32, unique=true)
     *
     * @Assert\NotBlank(groups={"registration"})
     */
    private $username;

    // ...
}

The @UniqueEntity constraint is being ignored. I tried different flavors including:

@UniqueEntity(fields={"email", "username"})

and

@UniqueEntity(fields={"email"})
@UniqueEntity(fields={"username"})

and, as per the Symfony2 documentation here: http://symfony.com/doc/current/reference/constraints/UniqueEntity.html

@UniqueEntity("email")
@UniqueEntity("username")

Nothing I do, works. Instead of getting a form validation error as expected, I am getting the following exception:

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'admin@scire.com' for key 'UNIQ_D8183973E7927C74'

This is just wrong! Does anyone know how to fix this issue?