JSON:API resources error: "This resource attempted to serve data that contains unsupported resource types." I'm trying to make a custom JSON:API resource for flaggings using the JSON:API Resources module.
Right now, I simply want to return all the flaggings of a given bundle with an EntityQuery.  (Later, I will customize the EntityQuery, which is why I am need to go beyond what JSON:API provides by default.)
My code is based on the module's example code.
MYMODULE/src/Resource/MyQueue.php
/**
 * Processes a request for a collection containing a user's article nodes.
 *
 * @internal
 */
class MyQueue extends EntityQueryResourceBase {

  /**
   * Process the resource request.
   *
   * Code example:
   * https://git.drupalcode.org/project/jsonapi_resources/tree/8.x-1.x/tests/modules/jsonapi_resources_test/src/Resource.
   *
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The request.
   *
   * @return \Drupal\jsonapi\ResourceResponse
   *   The response.
   *
   * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
   * @throws \Drupal\Component\Plugin\Exception\PluginNotFoundException
   */
  public function process(Request $request): ResourceResponse {

    $cacheability = new CacheableMetadata();

    $entity_type_id = 'flagging';
    $entity_type_bundle = 'MY_FLAGGING_TYPE';
    $entity_type = $this-&gt;entityTypeManager-&gt;getDefinition($entity_type_id);
    $entity_query = $this-&gt;getEntityQuery($entity_type_id)
      -&gt;condition($entity_type-&gt;getKey('bundle'), $entity_type_bundle);

    $paginator = $this-&gt;getPaginatorForRequest($request);
    $paginator-&gt;applyToQuery($entity_query, $cacheability);

    $data = $this-&gt;loadResourceObjectDataFromEntityQuery($entity_query, $cacheability);

    $pagination_links = $paginator-&gt;getPaginationLinks($entity_query, $cacheability, TRUE);

    $response = $this-&gt;createJsonapiResponse($data, $request, 200, [], $pagination_links);
    $response-&gt;addCacheableDependency($cacheability);

    return $response;
  }

}

With this code, I'm getting the following error:

LogicException: This resource attempted to serve data that contains
unsupported resource types.

I don't understand why I'm getting this error, or what it means.

If I set $entity_type_id to node and $entity_type_bundle to page, everything works as expected.
If I set the $entity_type_id to 'custom_content_type`, one of the content types I configured in the admin UI, it doesn't work; I get the same error.
I tried to set $entity_type_bundle to all three flagging bundles on my site; none of them work.
The $entity_query is correct for the flaggings.  I can do $entity_query-&gt;count-&gt;execute() and I get the correct count.
I am logged in as an administrator with all permissions.
I can access the jsonapi in my browser as an admin at /jsonapi/flagging/MY_FLAGGING_TYPE and I see the flaggings as I expect.
This error is the result of not setting the _jsonapi_resource correctly in the routing file:
MYMODULE.routing.yml
# Defines a route where a user's flaggings can be found.
MYMODULE.user_flaggings:
  # %jsonapi% is a placeholder for the JSON:API base path, which can be
  # configured in a site's services.yml file.
  path: '/%jsonapi%/MYMODULE/queue'
  defaults:
    # Every JSON:API resource route must declare a _jsonapi_resource. The
    # value can either be a class or a service ID. Unlike the _controller
    # route default, it is not possible to declare a method name to be called.
    _jsonapi_resource: Drupal\MYMODULE\Resource\MyQueue
    _jsonapi_resource_types: ['node--page', 'flagging--MY_FLAGGING_TYPE']