diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index a0e3dd6..4cd4bd7 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-version: ['7.4', '8.0', '8.1', '8.2', '8.3']
+        php-version: ['7.4', '8.0', '8.1', '8.2', '8.3', '8.4']
         os: [ubuntu-latest]
     runs-on: ${{ matrix.os }}
 
diff --git a/src/Amplitude/Amplitude.php b/src/Amplitude/Amplitude.php
index 7746d6d..da8d45e 100644
--- a/src/Amplitude/Amplitude.php
+++ b/src/Amplitude/Amplitude.php
@@ -26,7 +26,7 @@ class Amplitude
     private LoggerInterface $logger;
     private AmplitudeConfig $config;
 
-    public function __construct(string $apiKey, AmplitudeConfig $config = null)
+    public function __construct(string $apiKey, ?AmplitudeConfig $config = null)
     {
         $this->apiKey = $apiKey;
         $this->config = $config ?? AmplitudeConfig::builder()->build();
diff --git a/tests/Amplitude/MockAmplitude.php b/tests/Amplitude/MockAmplitude.php
index 6a0d6e3..966b6ed 100644
--- a/tests/Amplitude/MockAmplitude.php
+++ b/tests/Amplitude/MockAmplitude.php
@@ -9,7 +9,7 @@
 
 class MockAmplitude extends Amplitude
 {
-    public function __construct(string $apiKey, AmplitudeConfig $config = null)
+    public function __construct(string $apiKey, ?AmplitudeConfig $config = null)
     {
         parent::__construct($apiKey, $config);
     }