diff --git a/.github/workflows/tests.yaml b/.github/workflows/tests.yaml
index 056cd8c5..86d1fe72 100644
--- a/.github/workflows/tests.yaml
+++ b/.github/workflows/tests.yaml
@@ -18,6 +18,7 @@ jobs:
       fail-fast: false
       matrix:
         php:
+          - '8.1'
           - '8.0'
           - '7.4'
           - '7.3'
diff --git a/tests/End2End/App/Controller/MainController.php b/tests/End2End/App/Controller/MainController.php
index bf0e5942..a8848838 100644
--- a/tests/End2End/App/Controller/MainController.php
+++ b/tests/End2End/App/Controller/MainController.php
@@ -41,7 +41,7 @@ public function exception(): Response
 
     public function fatal(): Response
     {
-        $foo = eval("return new class() implements \Serializable {};");
+        $foo = eval("return new class() implements \JsonSerializable {};");
 
         return new Response('This response should not happen: ' . json_encode($foo));
     }
diff --git a/tests/End2End/End2EndTest.php b/tests/End2End/End2EndTest.php
index 37df19e1..b6211b9c 100644
--- a/tests/End2End/End2EndTest.php
+++ b/tests/End2End/End2EndTest.php
@@ -151,7 +151,7 @@ public function testGetFatal(): void
             $this->assertStringNotContainsString('not happen', $response->getContent() ?: '');
         } catch (\RuntimeException $exception) {
             $this->assertStringContainsStringIgnoringCase('error', $exception->getMessage());
-            $this->assertStringContainsStringIgnoringCase('contains 2 abstract methods', $exception->getMessage());
+            $this->assertStringContainsStringIgnoringCase('contains 1 abstract method', $exception->getMessage());
             $this->assertStringContainsStringIgnoringCase('MainController.php', $exception->getMessage());
             $this->assertStringContainsStringIgnoringCase('eval()\'d code on line', $exception->getMessage());
         }