diff --git a/core/lib/Drupal/Core/Config/ConfigEvents.php b/core/lib/Drupal/Core/Config/ConfigEvents.php
index dd60540..6edbddc 100644
--- a/core/lib/Drupal/Core/Config/ConfigEvents.php
+++ b/core/lib/Drupal/Core/Config/ConfigEvents.php
@@ -21,13 +21,9 @@
    * object is saved. The event listener method receives a
    * \Drupal\Core\Config\ConfigCrudEvent instance.
    *
-   * This event will be fired when configuration is saved by hook_update_N()
-   * implementations. Subscribers to this event can not make any assumption that
-   * the config data is valid and should ensure that their actions are safe
-   * regardless of the structure of the underlying configuration. When changing
-   * configuration values, the value must be the correct data type.
-   * Configuration must be saved with the $trusted_data flag set to TRUE so that
-   * configuration schema are not used.
+   * See hook_update_N() documentation for safe configuration API usage and
+   * restrictions as this event will be fired when configuration is saved by
+   * hook_update_N().
    *
    * @Event
    *
@@ -47,13 +43,9 @@
    * object is deleted. The event listener method receives a
    * \Drupal\Core\Config\ConfigCrudEvent instance.
    *
-   * This event will be fired when configuration is deleted by hook_update_N()
-   * implementations. Subscribers to this event can not make any assumption that
-   * the config data is valid and should ensure that their actions are safe
-   * regardless of the structure of the underlying configuration. When changing
-   * configuration values, the value must be the correct data type.
-   * Configuration must be saved with the $trusted_data flag set to TRUE so that
-   * configuration schema are not used.
+   * See hook_update_N() documentation for safe configuration API usage and
+   * restrictions as this event will be fired when configuration is deleted by
+   * hook_update_N().
    *
    * @Event
    *
@@ -73,13 +65,9 @@
    * object's name is changed. The event listener method receives a
    * \Drupal\Core\Config\ConfigRenameEvent instance.
    *
-   * This event will be fired when configuration is renamed by hook_update_N()
-   * implementations. Subscribers to this event can not make any assumption that
-   * the config data is valid and should ensure that their actions are safe
-   * regardless of the structure of the underlying configuration. When changing
-   * configuration values, the value must be the correct data type.
-   * Configuration must be saved with the $trusted_data flag set to TRUE so that
-   * configuration schema are not used.
+   * See hook_update_N() documentation for safe configuration API usage and
+   * restrictions as this event will be fired when configuration is renamed by
+   * hook_update_N().
    *
    * @Event
    *
diff --git a/core/lib/Drupal/Core/Extension/module.api.php b/core/lib/Drupal/Core/Extension/module.api.php
index 55c2741..119ab7b 100644
--- a/core/lib/Drupal/Core/Extension/module.api.php
+++ b/core/lib/Drupal/Core/Extension/module.api.php
@@ -474,14 +474,19 @@ function hook_install_tasks_alter(&$tasks, $install_state) {
  * The following actions are examples that are safe:
  * - Cache invalidation.
  * - Using \Drupal::configFactory()->getEditable() and \Drupal::config().
- *   Implementations can not make any assumption that the config data is valid.
- *   Implementations must use the $trusted_data argument for
- *   \Drupal\Core\Config\Config::save() and pass schema correct data so that
- *   configuration schema are not used whilst saving configuration.
+ *   Implementations must:
+ *   - not make any assumption that the config data is valid.
+ *   - use be the correct data type when changing configuration values as
+ *     specified by its configuration schema at the time the update hook is
+ *     written. If the data type changes in a subsequent code change, a
+ *     subsequent update hook is responsible for ensuring the final data type
+ *     aligns with the configuration schema.
+ *   - use the $has_trusted_data argument for \Drupal\Core\Config\Config::save()
+ *     so that configuration schemas are not used whilst saving configuration.
  * - Marking a container for rebuild.
  *
  * The following actions are examples that are unsafe:
- * - Calling Node::save().
+ * - Loading, saving, or performing any other operation on an entity.
  * - Rebuilding the router using \Drupal::service('router.builder')->rebuild().
  *
  * The $sandbox parameter should be used when a multipass update is needed, in
diff --git a/core/modules/language/language.services.yml b/core/modules/language/language.services.yml
index fc9bba1..62ca19c 100644
--- a/core/modules/language/language.services.yml
+++ b/core/modules/language/language.services.yml
@@ -9,7 +9,7 @@ services:
       - [initLanguageManager]
   language.config_subscriber:
     class: Drupal\language\EventSubscriber\ConfigSubscriber
-    arguments: ['@language_manager', '@language.default', '@config.factory']
+    arguments: ['@language_manager', '@language.default', '@config.factory', '@config.typed']
     tags:
       - { name: event_subscriber }
   language.config_factory_override:
diff --git a/core/modules/language/src/EventSubscriber/ConfigSubscriber.php b/core/modules/language/src/EventSubscriber/ConfigSubscriber.php
index 4f087e3..738a1b0 100644
--- a/core/modules/language/src/EventSubscriber/ConfigSubscriber.php
+++ b/core/modules/language/src/EventSubscriber/ConfigSubscriber.php
@@ -8,6 +8,7 @@
 namespace Drupal\language\EventSubscriber;
 
 use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Config\TypedConfigManagerInterface;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Language\LanguageDefault;
 use Drupal\Core\Language\LanguageManagerInterface;
@@ -43,6 +44,13 @@ class ConfigSubscriber implements EventSubscriberInterface {
   protected $configFactory;
 
   /**
+   * The typed config manager.
+   *
+   * @var \Drupal\Core\Config\TypedConfigManagerInterface
+   */
+  protected $typedConfig;
+
+  /**
    * Constructs a new class object.
    *
    * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
@@ -51,28 +59,44 @@ class ConfigSubscriber implements EventSubscriberInterface {
    *   The default language.
    * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
    *   The configuration factory.
+   * @param \Drupal\Core\Config\TypedConfigManagerInterface $typed_config
+   *   The typed config manager.
    */
-  public function __construct(LanguageManagerInterface $language_manager, LanguageDefault $language_default, ConfigFactoryInterface $config_factory) {
+  public function __construct(LanguageManagerInterface $language_manager, LanguageDefault $language_default, ConfigFactoryInterface $config_factory, TypedConfigManagerInterface $typed_config) {
     $this->languageManager = $language_manager;
     $this->languageDefault = $language_default;
     $this->configFactory = $config_factory;
+    $this->typedConfig = $typed_config;
   }
 
   /**
    * Causes the container to be rebuilt on the next request.
    *
+   * This event assumes that the new default langcode and old default langcode
+   * are valid langcodes. If the schema definition of either
+   * system.site:default_langcode or language.negotiation::url.prefixes changes
+   * then this event must changed to work with both the old and new schema
+   * definition so this event is update safe.
+   *
    * @param ConfigCrudEvent $event
    *   The configuration event.
    */
   public function onConfigSave(ConfigCrudEvent $event) {
     $saved_config = $event->getConfig();
     if ($saved_config->getName() == 'system.site' && $event->isChanged('default_langcode')) {
+      // Ensure that the configuration that will be changed or relied upon has
+      // the expected data type of 'string'.
+      $default_langcode_definition = $this->typedConfig->get('system.site')->get('default_langcode')->getDataDefinition();
+      $old_default_langcode = $saved_config->getOriginal('default_langcode');
+      $url_prefix_definition = $this->typedConfig->get('language.negotiation')->get('url.prefixes.' . $old_default_langcode)->getDataDefinition();
+      if ($default_langcode_definition->getDataType() !== 'string' || $url_prefix_definition->getDataType() != 'string') {
+        return;
+      }
       $new_default_langcode = $saved_config->get('default_langcode');
 
       $default_language = $this->configFactory->get('language.entity.' . $new_default_langcode);
       // During an import the language might not exist yet.
       if (!$default_language->isNew()) {
-        $old_default_langcode = $saved_config->getOriginal('default_langcode');
         $this->languageDefault->set(new Language($default_language->get()));
         $this->languageManager->reset();