diff --git a/core/lib/Drupal/Core/Routing/RoutePreloader.php b/core/lib/Drupal/Core/Routing/RoutePreloader.php
index d0050c3..daa4130 100644
--- a/core/lib/Drupal/Core/Routing/RoutePreloader.php
+++ b/core/lib/Drupal/Core/Routing/RoutePreloader.php
@@ -98,7 +98,7 @@ protected function loadNonAdminRoutes() {
   public function onAlterRoutes(RouteBuildEvent $event) {
     $collection = $event->getRouteCollection();
     foreach ($collection->all() as $name => $route) {
-      if (strpos($route->getPath(), '/admin') !== 0) {
+      if (strpos($route->getPath(), '/admin/') !== 0 && $route->getPath() != '/admin') {
         $this->nonAdminRoutesOnRebuild[] = $name;
       }
     }
diff --git a/core/tests/Drupal/Tests/Core/Routing/RoutePreloaderTest.php b/core/tests/Drupal/Tests/Core/Routing/RoutePreloaderTest.php
index 3154d36..334e903 100644
--- a/core/tests/Drupal/Tests/Core/Routing/RoutePreloaderTest.php
+++ b/core/tests/Drupal/Tests/Core/Routing/RoutePreloaderTest.php
@@ -103,13 +103,15 @@ public function testOnAlterRoutesWithAdminPathNoAdminRoute() {
     $route_collection = new RouteCollection();
     $route_collection->add('test', new Route('/foo/admin/foo', array('_content' => 'Drupal\ExampleController')));
     $route_collection->add('test2', new Route('/bar/admin/bar', array('_content' => 'Drupal\ExampleController')));
+    $route_collection->add('test3', new Route('/administrator/a', array('_content' => 'Drupal\ExampleController')));
+    $route_collection->add('test4', new Route('/admin', array('_content' => 'Drupal\ExampleController')));
     $event->expects($this->once())
       ->method('getRouteCollection')
       ->will($this->returnValue($route_collection));
 
     $this->state->expects($this->once())
       ->method('set')
-      ->with('routing.non_admin_routes', array('test', 'test2'));
+      ->with('routing.non_admin_routes', array('test', 'test2', 'test3'));
     $this->preloader->onAlterRoutes($event);
     $this->preloader->onFinishedRoutes(new Event());
   }