diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 64e01e6..5f9de04 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -731,6 +731,7 @@ protected function buildFormAction() {
     //   https://www.drupal.org/node/2504709.
     $parsed = UrlHelper::parse($request_uri);
     unset($parsed['query'][static::AJAX_FORM_REQUEST], $parsed['query'][MainContentViewSubscriber::WRAPPER_FORMAT]);
+    $parsed['path'] = '/' . ltrim($parsed['path'], '/');
     return $parsed['path'] . ($parsed['query'] ? ('?' . UrlHelper::buildQuery($parsed['query'])) : '');
   }
 
diff --git a/core/modules/system/src/Tests/Form/ExternalFormUrlTest.php b/core/modules/system/src/Tests/Form/ExternalFormUrlTest.php
index a92e047..35d22f78 100644
--- a/core/modules/system/src/Tests/Form/ExternalFormUrlTest.php
+++ b/core/modules/system/src/Tests/Form/ExternalFormUrlTest.php
@@ -16,7 +16,7 @@
 use Symfony\Component\HttpFoundation\Request;
 
 /**
- * Ensures that form actions can't be tricked into sending to external urls.
+ * Ensures that form actions can't be tricked into sending to external URLs.
  *
  * @group system
  */
@@ -86,12 +86,9 @@ public function testExternalUrlBehavior() {
     $form = \Drupal::formBuilder()->getForm($this);
     $markup = \Drupal::service('renderer')->renderRoot($form);
 
-    $dom = new \DOMDocument();
-    $dom->loadXML($markup);
-
-    $xpath = new \DOMXPath($dom);
-    $form = $xpath->query('//form');
-    $action = $form->item(0)->getAttribute('action');
+    $this->setRawContent($markup);
+    $elements = $this->xpath('//form/@action');
+    $action = (string) $elements[0];
 
     $this->assertNotEqual('//example.org', $action);
     $this->assertEqual('/example.org', $action);