diff --git a/core/lib/Drupal/Core/Config/Schema/Element.php b/core/lib/Drupal/Core/Config/Schema/Element.php
index 2dc1056..54a4882 100644
--- a/core/lib/Drupal/Core/Config/Schema/Element.php
+++ b/core/lib/Drupal/Core/Config/Schema/Element.php
@@ -46,7 +46,10 @@ public function getTypedDataManager() {
    * ensure that only typed configuration manager can be used.
    *
    * @param \Drupal\Core\TypedData\TypedDataManagerInterface $typed_data_manager
-   *   The typed data manager.
+   *   The typed config manager. This must be an instance of
+   *   \Drupal\Core\Config\TypedConfigManagerInterface. If it is not, then this
+   *   method will error when assertions are enabled. We can not narrow the
+   *   typehint as this will cause PHP errors.
    *
    * @return $this
    */
diff --git a/core/lib/Drupal/Core/TypedData/TypedDataManagerInterface.php b/core/lib/Drupal/Core/TypedData/TypedDataManagerInterface.php
index 58bba2c..8f8ff99 100644
--- a/core/lib/Drupal/Core/TypedData/TypedDataManagerInterface.php
+++ b/core/lib/Drupal/Core/TypedData/TypedDataManagerInterface.php
@@ -15,7 +15,7 @@
 /**
  * Defines an interface for typed data manager.
  */
-Interface TypedDataManagerInterface extends PluginManagerInterface, CachedDiscoveryInterface {
+interface TypedDataManagerInterface extends PluginManagerInterface, CachedDiscoveryInterface {
 
   /**
    * {@inheritdoc}