Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" [error] plugin does not exist.

I realize this version is on hold, but just an FYI and possible future reference if you don't already know.

When installing via drush I get the following messages and error:

mac-admin:docroot lxt$ drush en administerusersbyrole
The following extensions will be enabled: administerusersbyrole
Do you really want to continue? (y/n): y
administerusersbyrole was enabled successfully. [ok]
Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" [error]
plugin does not exist.

I tracked it down to an epty class in: src/Annotation/AccessManager.php

<?php

namespace Drupal\administerusersbyrole\Annotation;

use Drupal\Component\Annotation\Plugin;

/**
* Defines an AliasType annotation.
*
* @Annotation
*/
class AccessManager extends Plugin {

}

Thanks!!