diff -u b/core/includes/menu.inc b/core/includes/menu.inc
--- b/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -16,6 +16,7 @@
 use Drupal\menu_link\MenuLinkStorageController;
 use Symfony\Cmf\Component\Routing\RouteObjectInterface;
 use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\Routing\Exception\RouteNotFoundException;
 use Symfony\Component\Routing\Route;
 
 /**
@@ -2554,21 +2555,44 @@
 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;
+  }
+
+  $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.', $machine_name));
-  }
+    if (!isset($link['link_title'])) {
+      throw new \Exception(sprintf('Menu link %s does not have a "link_title" key in its definition.', $machine_name));
+    }
 
-  // Check if the specified parent menu link exists.
+    // Check if the specified parent menu link exists.
     if (isset($link['parent']) && !isset($all_links[$link['parent']])) {
       throw new \OutOfBoundsException(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);
+      }
+    }
   }
-  $module_handler->alter('menu_link_defaults', $all_links);
+
   return $all_links;
 }