diff --git a/.github/workflows/run-tests.yml b/.github/workflows/run-tests.yml
index 377549d..2be929b 100644
--- a/.github/workflows/run-tests.yml
+++ b/.github/workflows/run-tests.yml
@@ -14,7 +14,7 @@ jobs:
       matrix:
         os: [ ubuntu-latest ]
         php: [ 8.0, 8.1 ]
-        stability: [ prefer-lowest, prefer-stable ]
+        stability: [ prefer-stable ]
 
     name: P${{ matrix.php }} - ${{ matrix.stability }} - ${{ matrix.os }}
 
diff --git a/.github/workflows/static-analysis.yml b/.github/workflows/static-analysis.yml
index 4c0dd73..39a3ac6 100644
--- a/.github/workflows/static-analysis.yml
+++ b/.github/workflows/static-analysis.yml
@@ -14,7 +14,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php: [ 8.0 ]
+        php: [ 8.1 ]
         os: [ ubuntu-latest ]
 
     steps:
diff --git a/src/PayloadSerializer.php b/src/PayloadSerializer.php
index 7bb53f4..9beaa67 100644
--- a/src/PayloadSerializer.php
+++ b/src/PayloadSerializer.php
@@ -19,7 +19,7 @@ public function __construct(
 
     public function serialize(Payload $payload): string
     {
-        return $this->serializer->serialize($payload);
+        return (string) $this->serializer->serialize($payload);
     }
 
     public function unserialize(string $payload): Payload