diff --git a/core/includes/menu.inc b/core/includes/menu.inc
index 987d9d0..f65e84c 100644
--- a/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -6,11 +6,13 @@
  */
 
 use Drupal\Component\Utility\String;
+use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Template\Attribute;
 use Drupal\menu_link\MenuLinkInterface;
 use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\Routing\Exception\RouteNotFoundException;
 
 /**
  * @defgroup menu Menu and routing system
@@ -1006,11 +1008,49 @@ function _menu_link_save_recursive($controller, $machine_name, &$children, &$lin
 function menu_link_get_defaults() {
   $module_handler = \Drupal::moduleHandler();
   $all_links = $module_handler->invokeAll('menu_link_defaults');
+  /** @var \Drupal\Core\Routing\RouteProviderInterface $route_provider */
+  $route_provider = \Drupal::service('router.route_provider');
+
   // Fill in the machine name from the array key.
-  foreach ($all_links as $machine_name => &$link) {
-    $link['machine_name'] = $machine_name;
+  foreach ($all_links as $machine_name => $link) {
+    $all_links[$machine_name]['machine_name'] = $machine_name;
   }
   $module_handler->alter('menu_link_defaults', $all_links);
+
+  // Validate the menu link definitions.
+  foreach ($all_links as $machine_name => $link) {
+
+    // Check for required array keys.
+    if (!isset($link['link_title'])) {
+      throw new \Exception(sprintf('Menu link %s does not have a "link_title" key in its definition.', $machine_name));
+    }
+
+    // Check that the link path is external.
+    if (isset($link['link_path']) && UrlHelper::isValid($link['link_path'], TRUE)) {
+      throw new \Exception(sprintf('link_path %s for menu link %s must be absolute.', $link['link_path'], $machine_name));
+    }
+
+    // Check if the specified parent menu link exists.
+    if (isset($link['parent']) && !isset($all_links[$link['parent']])) {
+      throw new \Exception(sprintf('Menu link %s specifies it has parent %s, but no menu link with that machine name exists.', $machine_name, $link['parent']));
+    }
+
+    // Check for either a route name or a link path.
+    if (!isset($link['route_name']) && !isset($link['link_path'])) {
+      throw new \Exception(sprintf('Menu link %s must provide either a "route_name" or a "link_path" key in its definition.', $machine_name));
+    }
+
+    // Check if the specified route exists.
+    if (isset($link['route_name'])) {
+      try {
+        $route_provider->getRouteByName($link['route_name']);
+      }
+      catch (RouteNotFoundException $e) {
+        throw new \Exception(sprintf('Menu link %s specifies route name %s, but no route with that machine name exists.', $machine_name, $link['route_name']), 0, $e);
+      }
+    }
+  }
+
   return $all_links;
 }
 
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 8083f89..c136826 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -429,7 +429,7 @@ function hook_page_build(&$page) {
  *   The value corresponding to each machine name key is an associative array
  *   that may contain the following key-value pairs:
  *   - link_title: (required) The untranslated title of the menu item.
- *   - description: The untranslated description of the link.
+ *   - description: (optional) The untranslated description of the link.
  *   - route_name: (optional) The route name to be used to build the path.
  *     Either a route_name or a link_path must be provided.
  *   - route_parameters: (optional) The route parameters to build the path.