Error in settings form

The following error happens on the module settings form:

User error: "ROLENAME" is an invalid render array key in Drupal\Core\Render\Element::children() (line 97 of core/lib/Drupal/Core/Render/Element.php). 

It's caused by a typo in the buildForm() method:

       '#options' => $roles_options,
-      '$default_value' => $config->get('role_watchdog_monitor_roles') != NULL ? $config->get('role_watchdog_monitor_roles') : NULL,
+      '#default_value' => $config->get('role_watchdog_monitor_roles') != NULL ? $config->get('role_watchdog_monitor_roles') : NULL,
       '#description' => $this->t('Select roles to monitor for change.'),