diff --git a/core/modules/rest/tests/src/Unit/EventSubscriber/ResourceResponseSubscriberTest.php b/core/modules/rest/tests/src/Unit/EventSubscriber/ResourceResponseSubscriberTest.php
index 9d7737d..5cf2953 100644
--- a/core/modules/rest/tests/src/Unit/EventSubscriber/ResourceResponseSubscriberTest.php
+++ b/core/modules/rest/tests/src/Unit/EventSubscriber/ResourceResponseSubscriberTest.php
@@ -89,7 +89,9 @@ public function testResponseFormat($methods, array $supported_formats, $request_
       $request = Request::create('/rest/test', $method, [], [], [], $request_headers, $request_body);
       // \Drupal\Core\StackMiddleware\NegotiationMiddleware normally takes care
       // of this so we'll hard code it here.
-      $request->setRequestFormat($request_format);
+      if ($request_format) {
+        $request->setRequestFormat($request_format);
+      }
       $route_requirement_key_format = $request->isMethodSafe() ? '_format' : '_content_type_format';
       $route_match = new RouteMatch('test', new Route('/rest/test', ['_rest_resource_config' => $this->randomMachineName()], [$route_requirement_key_format => implode('|', $supported_formats)]));
 
@@ -124,7 +126,9 @@ public function testOnResponseWithCacheableResponse($methods, array $supported_f
       $request = Request::create('/rest/test', $method, [], [], [], $request_headers, $request_body);
       // \Drupal\Core\StackMiddleware\NegotiationMiddleware normally takes care
       // of this so we'll hard code it here.
-      $request->setRequestFormat($request_format);
+      if ($request_format) {
+        $request->setRequestFormat($request_format);
+      }
       $route_requirement_key_format = $request->isMethodSafe() ? '_format' : '_content_type_format';
       $route_match = new RouteMatch('test', new Route('/rest/test', ['_rest_resource_config' => $rest_config_name], [$route_requirement_key_format => implode('|', $supported_formats)]));
 
@@ -172,7 +176,9 @@ public function testOnResponseWithUncacheableResponse($methods, array $supported
       $request = Request::create('/rest/test', $method, [], [], [], $request_headers, $request_body);
       // \Drupal\Core\StackMiddleware\NegotiationMiddleware normally takes care
       // of this so we'll hard code it here.
-      $request->setRequestFormat($request_format);
+      if ($request_format) {
+        $request->setRequestFormat($request_format);
+      }
       $route_requirement_key_format = $request->isMethodSafe() ? '_format' : '_content_type_format';
       $route_match = new RouteMatch('test', new Route('/rest/test', ['_rest_resource_config' => $rest_config_name], [$route_requirement_key_format => implode('|', $supported_formats)]));