diff --git a/core/tests/Drupal/Tests/Core/UnroutedUrlTest.php b/core/tests/Drupal/Tests/Core/UnroutedUrlTest.php
index 1b3003f..8503ad5 100644
--- a/core/tests/Drupal/Tests/Core/UnroutedUrlTest.php
+++ b/core/tests/Drupal/Tests/Core/UnroutedUrlTest.php
@@ -87,6 +87,8 @@ public function providerFromUri() {
       // [$uri, $is_external]
       // An external URI.
       ['https://www.drupal.org', TRUE],
+      // A protocol relative URL.
+      ['//www.drupal.org', TRUE],
       // An internal, unrouted, base-relative URI.
       ['base:robots.txt', FALSE],
       // Base-relative URIs with special characters.