Ограничение UniqueEntity не работает

Я пытаюсь использовать ограничение уникальной сущности в своей пользовательской сущности, но почему-то это не работает. Я по-прежнему получаю сообщение об ошибке дублирования ключа.

Это мой пользовательский объект:

use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;

/**
 * @ORM\Table(name = "Users")
 * @ORM\Entity(repositoryClass = "App\Repository\UserRepository")
 * @UniqueEntity(
 *     fields = {"email", "username"}
 * )
 */
class User implements AdvancedUserInterface, \Serializable
{
    /**
     * @ORM\Column(type = "integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy = "AUTO")
     */
    public $id;

    /**
     * @ORM\Column(type = "string", length=25, unique=true)
     */
    public $username;

    /**
     * @ORM\Column(type = "string", length=64)
     */
    public $password;

    /**
     * @ORM\Column(type = "string", length=128, unique=true)
     * @Assert\Email()
     */
    public $email;

    /**
     * @ORM\Column(name = "is_active", type = "boolean")
     */
    public $isActive;

    /**
     * @ORM\Column(name = "roles", type = "array")
     */
    public $roles = [];

    /**
     * @ORM\OneToOne(targetEntity = "App\Entity\UserDetails", mappedBy = "user", cascade = {"persist", "remove"})
     */
    private $userDetails;

    /**
     * @ORM\ManyToMany(targetEntity = "App\Entity\Assignment", mappedBy = "user")
     */
    private $assignments;

    public function __construct()
    {
        $this->isActive = true;
        $this->assignments = new ArrayCollection();
        // may not be needed, see section on salt below
        // $this->salt = md5(uniqid('', true));
    }

    /**
     * @param mixed $isActive
     */
    public function setIsActive($isActive)
    {
        $this->isActive = $isActive;
    }


    public function getEmail()
    {
        return $this->email;
    }

    public function setEmail($email)
    {
        $this->email = $email;
    }

    public function getUsername()
    {
        return $this->username;
    }

    public function setUsername($username)
    {
        $this->username = $username;
    }

    public function getPassword()
    {
        return $this->password;
    }

    public function setPassword($password)
    {
        $this->password = $password;
    }

    public function getSalt()
    {
        // The bcrypt and argon2i algorithms don't require a separate salt.
        // You *may* need a real salt if you choose a different encoder.
        return null;
    }


    public function getRoles()
    {
        $roles = $this->roles;

        if (!in_array('ROLE_USER', $roles)) {
            $roles[] = 'ROLE_USER';
        }
        return $roles;
    }

    /**
     * @return mixed
     */
    public function getId()
    {
        return $this->id;
    }

    public function setRoles(array $roles)
    {
        $this->roles = $roles;
    }

    public function eraseCredentials()
    {
    }

    /** @see \Serializable::serialize() */
    public function serialize()
    {
        return serialize(array(
            $this->id,
            $this->username,
            $this->password,
            $this->isActive
            // see section on salt below
            // $this->salt,
        ));
    }

    /** @see \Serializable::unserialize() */
    public function unserialize($serialized)
    {
        list (
            $this->id,
            $this->username,
            $this->password,
            $this->isActive
            // see section on salt below
            // $this->salt
            ) = unserialize($serialized, ['allowed_classes' => false]);
    }

    public function isAccountNonExpired()
    {
        return true;
    }

    public function isAccountNonLocked()
    {
        return true;
    }

    public function isCredentialsNonExpired()
    {
        return true;
    }

    public function isEnabled()
    {
        return $this->isActive;
    }

    public function getUserDetails(): ?UserDetails
    {
        return $this->userDetails;
    }

    public function setUserDetails(UserDetails $userDetails): self
    {
        $this->userDetails = $userDetails;

        // set the owning side of the relation if necessary
        if ($this !== $userDetails->getUser()) {
            $userDetails->setUser($this);
        }

        return $this;
    }

    /**
     * @return Collection|Assignment[]
     */
    public function getAssignments(): Collection
    {
        return $this->assignments;
    }

    public function addAssignment(Assignment $assignment): self
    {
        if (!$this->assignments->contains($assignment)) {
            $this->assignments[] = $assignment;
            $assignment->addUser($this);
        }

        return $this;
    }

    public function removeAssignment(Assignment $assignment): self
    {
        if ($this->assignments->contains($assignment)) {
            $this->assignments->removeElement($assignment);
            $assignment->removeUser($this);
        }

        return $this;
    }
}

При отправке формы я получаю следующее сообщение об ошибке:

An exception occurred while executing 'INSERT INTO Users (username, password, email, is_active, roles) VALUES (?, ?, ?, ?, ?)' with params ["sanderbakker", "$2y$13$evlC1NcG9BIdgeBRuBqCKu4uqt4Edsp24SEUIiYF3vg6rp0tEhHdm", "[email protected]", 1, "a:1:{i:0;s:10:\"ROLE_ADMIN\";}"]:

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'sanderbakker' for key 'UNIQ_D5428AEDF85E0677'

Эта ошибка не появится, если я использую только один атрибут в качестве ограничения. Например, только атрибут имени пользователя или атрибут электронной почты