diff --git a/core/modules/rest/tests/src/Functional/CookieResourceTestTrait.php b/core/modules/rest/tests/src/Functional/CookieResourceTestTrait.php
index 18dc296..41fde93 100644
--- a/core/modules/rest/tests/src/Functional/CookieResourceTestTrait.php
+++ b/core/modules/rest/tests/src/Functional/CookieResourceTestTrait.php
@@ -64,7 +64,10 @@ protected function initAuthentication() {
     ];
 
     $request_options[RequestOptions::BODY] = $this->serializer->encode($request_body, 'json');
-    $request_options[RequestOptions::HEADERS]['Accept'] = 'application/json';
+    $request_options[RequestOptions::HEADERS] = [
+      'Accept' => 'application/json',
+      'Content-Type' => 'application/json',
+    ];
     $response = $this->request('POST', $user_login_url, $request_options);
 
     // Parse and store the session cookie.