diff --git a/core/lib/Drupal/Core/EventSubscriber/PathRootsSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/PathRootsSubscriber.php
index 824c4e89be3..a0bd7b06400 100644
--- a/core/lib/Drupal/Core/EventSubscriber/PathRootsSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/PathRootsSubscriber.php
@@ -20,7 +20,7 @@ class PathRootsSubscriber implements EventSubscriberInterface {
    *
    * @var array
    */
-  protected $pathRoots;
+  protected $pathRoots = [];
 
   /**
    * The state key value store.
@@ -58,7 +58,7 @@ class PathRootsSubscriber implements EventSubscriberInterface {
    */
   public function onRouteFinished() {
     $this->state->set('router.path_roots', array_keys($this->pathRoots));
-    unset($this->pathRoots);
+    $this->pathRoots = [];
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/EventSubscriber/PathRootsSubscriberTest.php b/core/tests/Drupal/Tests/Core/EventSubscriber/PathRootsSubscriberTest.php
index 9e77accfe7e..96a922829a1 100644
--- a/core/tests/Drupal/Tests/Core/EventSubscriber/PathRootsSubscriberTest.php
+++ b/core/tests/Drupal/Tests/Core/EventSubscriber/PathRootsSubscriberTest.php
@@ -43,6 +43,11 @@ class PathRootsSubscriberTest extends UnitTestCase {
    * @covers ::onRouteFinished
    */
   public function testSubscribing() {
+
+    // Ensure that onRouteFinished can be called without throwing notices
+    // when no path roots got set.
+    $this->pathRootsSubscriber->onRouteFinished();
+
     $route_collection = new RouteCollection();
     $route_collection->add('test_route1', new Route('/test/bar'));
     $route_collection->add('test_route2', new Route('/test/baz'));