Symfony – FOSUserBundle override mapping to remove need for username

I want to remove the need for a username in the FOSUserBundle. My users will login using an email address only and I've added real name fields as part of the user entity.
I realised that I needed to redo the entire mapping as described here.
I think I've done it correctly but when I try to submit the registration form I get the error:

"Only field names mapped by Doctrine can be validated for uniqueness."

The strange thing is that I haven't tried to assert a unique constraint to anything in the user entity.

Here is my full user entity file:

 <?php
        // src/MyApp/UserBundle/Entity/User.php

        namespace MyApp\UserBundle\Entity;

        use FOS\UserBundle\Model\User as BaseUser;
        use Doctrine\ORM\Mapping as ORM;
        use Symfony\Component\Validator\Constraints as Assert;

        /**
         * @ORM\Entity
         * @ORM\Table(name="depbook_user")
         */
        class User extends BaseUser
        {
            /**
             * @ORM\Id
             * @ORM\Column(type="integer")
             * @ORM\GeneratedValue(strategy="AUTO")
             */
            protected $id;

            /**
             * @ORM\Column(type="string", length=255)
             *
             * @Assert\NotBlank(message="Please enter your first name.", groups={"Registration", "Profile"})
             * @Assert\MaxLength(limit="255", message="The name is too long.", groups={"Registration", "Profile"})
             */
            protected $firstName;

            /**
             * @ORM\Column(type="string", length=255)
             *
             * @Assert\NotBlank(message="Please enter your last name.", groups={"Registration", "Profile"})
             * @Assert\MaxLength(limit="255", message="The name is too long.", groups={"Registration", "Profile"})
             */
            protected $lastName;

            /**
             * @ORM\Column(type="string", length=255)
             *
             * @Assert\NotBlank(message="Please enter your email address.", groups={"Registration", "Profile"})
             * @Assert\MaxLength(limit="255", message="The name is too long.", groups={"Registration", "Profile"})
             * @Assert\Email(groups={"Registration"})
             */
            protected $email;

            /**
             * @ORM\Column(type="string", length=255, name="email_canonical", unique=true)
             */
             protected $emailCanonical;

            /**
             * @ORM\Column(type="boolean")
             */
             protected $enabled;

            /**
             * @ORM\Column(type="string")
             */
             protected $salt;

            /**
             * @ORM\Column(type="string")
             */
             protected $password;

            /**
             * @ORM\Column(type="datetime", nullable=true, name="last_login")
             */
             protected $lastLogin;

            /**
             * @ORM\Column(type="boolean")
             */
             protected $locked;

            /**
             * @ORM\Column(type="boolean")
             */
             protected $expired;

            /**
             * @ORM\Column(type="datetime", nullable=true, name="expires_at")
             */
             protected $expiresAt;

            /**
             * @ORM\Column(type="string", nullable=true, name="confirmation_token")
             */
             protected $confirmationToken;

            /**
             * @ORM\Column(type="datetime", nullable=true, name="password_requested_at")
             */
             protected $passwordRequestedAt;

            /**
             * @ORM\Column(type="array")
             */
             protected $roles;

            /**
             * @ORM\Column(type="boolean", name="credentials_expired")
             */
             protected $credentialsExpired;

            /**
             * @ORM\Column(type="datetime", nullable=true, name="credentials_expired_at")
             */
             protected $credentialsExpiredAt;

            public function __construct()
            {
                parent::__construct();
                // your own logic
            }

            /**
             * @return string
             */
            public function getFirstName()
            {
                return $this->firstName;
            }

            /**
             * @return string
             */
            public function getLastName()
            {
                return $this->lastName;
            }

             /**
             * Sets the first name.
             *
             * @param string $firstname
             *
             * @return User
             */
            public function setFirstName($firstname)
            {
                $this->firstName = $firstname;

                return $this;
            }

                 /**
             * Sets the last name.
             *
             * @param string $lastname
             *
             * @return User
             */
            public function setLastName($lastname)
            {
                $this->lastName = $lastname;

                return $this;


       }
    }

I've seen various suggestions about this but none of the suggestions seem to work for me. The FOSUserBundle docs are very sparse about what must be a very common request.