diff --git a/core/lib/Drupal/Core/Routing/UrlGenerator.php b/core/lib/Drupal/Core/Routing/UrlGenerator.php
index a9a9e1a..5a58722 100644
--- a/core/lib/Drupal/Core/Routing/UrlGenerator.php
+++ b/core/lib/Drupal/Core/Routing/UrlGenerator.php
@@ -83,6 +83,8 @@ public function __construct(RouteProviderInterface $provider, OutboundPathProces
 
     $this->pathProcessor = $path_processor;
     $this->routeProcessor = $route_processor;
+    // Ensure to have a safe fallback. This is also needed for some tests, which
+    // don't specify the filter protocols container parameter.
     UrlHelper::setAllowedProtocols($filter_protocols ?: ['http', 'https']);
     $this->requestStack = $request_stack;
   }
diff --git a/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php b/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
index 550e1a4..b9e729a 100644
--- a/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
+++ b/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
@@ -46,6 +46,8 @@ class UnroutedUrlAssembler implements UnroutedUrlAssemblerInterface {
    *   An array of protocols allowed for URL generation.
    */
   public function __construct(RequestStack $request_stack, OutboundPathProcessorInterface $path_processor, $filter_protocols = []) {
+    // Ensure to have a safe fallback. This is also needed for some tests, which
+    // don't specify the filter protocols container parameter.
     UrlHelper::setAllowedProtocols($filter_protocols ?: ['http', 'https']);
     $this->requestStack = $request_stack;
     $this->pathProcessor = $path_processor;