diff --git a/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php b/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php
index 7a9ab61..0096257 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php
@@ -73,6 +73,8 @@ public function getDefinition($plugin_id, $exception_on_invalid = TRUE) {
         else {
           $plugin_definition = $derivative_plugin_definition;
         }
+        // It is vital that derivative plugin definitions contain derivative
+        // plugin IDs. Enforce it here, because not all derivers do it.
         $plugin_definition['id'] = $plugin_id;
       }
     }
@@ -111,6 +113,8 @@ protected function getDerivatives(array $base_plugin_definitions) {
           if ($derivative_id && isset($base_plugin_definitions[$plugin_id])) {
             $derivative_definition = $this->mergeDerivativeDefinition($base_plugin_definitions[$plugin_id], $derivative_definition);
           }
+          // It is vital that derivative plugin definitions contain derivative
+          // plugin IDs. Enforce it here, because not all derivers do it.
           $derivative_definition['id'] = $plugin_id;
           $plugin_definitions[$plugin_id] = $derivative_definition;
         }