diff --git a/core/lib/Drupal/Core/Routing/RequestHelper.php b/core/lib/Drupal/Core/Routing/RequestHelper.php
index d137e7f..b060e47 100644
--- a/core/lib/Drupal/Core/Routing/RequestHelper.php
+++ b/core/lib/Drupal/Core/Routing/RequestHelper.php
@@ -30,7 +30,7 @@ class RequestHelper {
    *   The query (GET) or request (POST) parameters.
    * @param array $query
    *   The GET parameters.
-   * @param array $request
+   * @param array $post
    *   The POST parameters.
    * @param array $attributes
    *   The request attributes (parameters parsed from the PATH_INFO, ...).
@@ -47,8 +47,8 @@ class RequestHelper {
    * @see \Symfony\Component\HttpFoundation\Request::create()
    * @see \Symfony\Component\HttpFoundation\Request::duplicate()
    */
-  public static function duplicate(Request $original_request, $uri, $method = 'GET', $parameters = array(), array $query = NULL, array $request = NULL, array $attributes = NULL, array $cookies = NULL, array $files = NULL, array $server = NULL) {
-    $request_clone = $original_request->duplicate($query, $request, $attributes, $cookies, $files, $server);
+  public static function duplicate(Request $original_request, $uri, $method = 'GET', $parameters = array(), array $query = NULL, array $post = NULL, array $attributes = NULL, array $cookies = NULL, array $files = NULL, array $server = NULL) {
+    $request = $original_request->duplicate($query, $post, $attributes, $cookies, $files, $server);
 
     $server = array();
 
@@ -97,11 +97,11 @@ public static function duplicate(Request $original_request, $uri, $method = 'GET
           $server['CONTENT_TYPE'] = 'application/x-www-form-urlencoded';
         }
       case 'PATCH':
-        $request = $parameters;
+        $post = $parameters;
         $query = array();
         break;
       default:
-        $request = array();
+        $post = array();
         $query = $parameters;
         break;
     }
@@ -119,12 +119,13 @@ public static function duplicate(Request $original_request, $uri, $method = 'GET
 
     $server['REQUEST_URI'] = $components['path'] . $query_string;
     $server['QUERY_STRING'] = $query_string;
-    $request_clone->server->add($server);
+    $request->server->add($server);
+    // The 'request' attribute name corresponds to $_REQUEST, but Symfony
+    // documents it as holding the POST parameters.
+    $request->request->add($post);
+    $request->query->add($query);
 
-    $request_clone->request->add($request);
-    $request_clone->query->add($query);
-
-    return $request_clone;
+    return $request;
   }
 
 }