ID mapping with generator overriding custom_id

Document.php

<?php

namespace My\Custom;

use Doctrine\ODM\MongoDB\Mapping\Annotations as ODM;

class Document {
  /**
   * This works
   * 
   * @ODM\Id(
   *     strategy="custom",
   *     options={
   *         "class"="My\Custom\MongoIdGenerator",
   *         "type"="id"
   *     }
   * )
   */
  protected $id;

  /**
   * This doesn't work
   * 
   * @ODM\Id(
   *     type="My\Custom\IdType",
   *     strategy="custom",
   *     options={
   *         "class"="My\Custom\MongoIdGenerator"
   *     }
   * )
   */
  protected $id;
}

document.xml

<?xml version="1.0" encoding="UTF-8"?>

<doctrine-mongo-mapping xmlns="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping"
                  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                  xsi:schemaLocation="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping
                  http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping.xsd">

    <document name="My\Custom\Document" db="documents" collection="documents">
        <!-- Works -->
        <field name="id" id="true" strategy="custom">
            <id-generator-option name="class" value="My\Custom\MongoIdGenerator"/>
            <id-generator-option name="type" value="id"/>
        </field>
        <!-- Doesn't work -->
        <field name="id" id="true" strategy="custom">
            <id-generator-option name="class" value="My\Custom\MongoIdGenerator"/>
        </field>
    </document>
</doctrine-mongo-mapping>

document.yml

My\Custom\Document:
  db: documents
  collection: documents
  fields:
    # Works
    id:
      fieldName: id
      id: true
      strategy: custom
      generatorOptions:
        class: My\Custom\MongoIdGenerator
        type: id
    # Doesn't work
    id:
      fieldName: id
      id: true
      strategy: custom
      generatorOptions:
        class: My\Custom\MongoIdGenerator