diff --git a/core/modules/rest/src/Tests/RESTTestBase.php b/core/modules/rest/src/Tests/RESTTestBase.php
index bb1a5f1..23d779c 100644
--- a/core/modules/rest/src/Tests/RESTTestBase.php
+++ b/core/modules/rest/src/Tests/RESTTestBase.php
@@ -8,6 +8,7 @@
 use Drupal\simpletest\WebTestBase;
 use GuzzleHttp\Cookie\CookieJar;
 use GuzzleHttp\Cookie\FileCookieJar;
+use GuzzleHttp\Cookie\SetCookie;
 
 /**
  * Test helper class that provides a REST client method to send HTTP requests.
@@ -125,8 +126,8 @@ protected function cookies() {
     }
 
     $cookie_jar = new FileCookieJar($this->cookieFile);
-    foreach ($cookies as $key => $cookie) {
-      $cookie_jar[$key] = $cookie;
+    foreach ($cookies as $key => $cookie_values) {
+      $cookie_jar->setCookie(new SetCookie(['Name' => $key, 'Value' => $cookie]));
     }
 
     return $cookie_jar;