diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 323c32e80e..9367a494c4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -53,7 +53,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-versions: [ '7.4', '8.0', '8.1', '8.2', '8.3']
+        php-versions: [ '7.4', '8.0', '8.1', '8.2', '8.3', '8.4' ]
         coverage: [ 'xdebug' ]
         streaming: [ false ]
         include:
diff --git a/lib/CardDAV/Plugin.php b/lib/CardDAV/Plugin.php
index c2d31d9dfe..810ae3a1ab 100644
--- a/lib/CardDAV/Plugin.php
+++ b/lib/CardDAV/Plugin.php
@@ -800,7 +800,7 @@ protected function negotiateVCard($input, &$mimeType = null)
      *
      * @return string
      */
-    protected function convertVCard($data, $target, array $propertiesFilter = null)
+    protected function convertVCard($data, $target, ?array $propertiesFilter = null)
     {
         if (is_resource($data)) {
             $data = stream_get_contents($data);
diff --git a/lib/CardDAV/Xml/Property/SupportedAddressData.php b/lib/CardDAV/Xml/Property/SupportedAddressData.php
index a912724bf9..c3cc7741d8 100644
--- a/lib/CardDAV/Xml/Property/SupportedAddressData.php
+++ b/lib/CardDAV/Xml/Property/SupportedAddressData.php
@@ -34,7 +34,7 @@ class SupportedAddressData implements XmlSerializable
     /**
      * Creates the property.
      */
-    public function __construct(array $supportedData = null)
+    public function __construct(?array $supportedData = null)
     {
         if (is_null($supportedData)) {
             $supportedData = [
diff --git a/lib/DAV/Auth/Plugin.php b/lib/DAV/Auth/Plugin.php
index eb4f27ca67..47fbe205ac 100644
--- a/lib/DAV/Auth/Plugin.php
+++ b/lib/DAV/Auth/Plugin.php
@@ -58,7 +58,7 @@ class Plugin extends ServerPlugin
      *
      * @param Backend\BackendInterface $authBackend
      */
-    public function __construct(Backend\BackendInterface $authBackend = null)
+    public function __construct(?Backend\BackendInterface $authBackend = null)
     {
         if (!is_null($authBackend)) {
             $this->addBackend($authBackend);
diff --git a/lib/DAV/Exception/Locked.php b/lib/DAV/Exception/Locked.php
index 28263cf13e..24fad70955 100644
--- a/lib/DAV/Exception/Locked.php
+++ b/lib/DAV/Exception/Locked.php
@@ -32,7 +32,7 @@ class Locked extends DAV\Exception
      *
      * @param DAV\Locks\LockInfo $lock
      */
-    public function __construct(DAV\Locks\LockInfo $lock = null)
+    public function __construct(?DAV\Locks\LockInfo $lock = null)
     {
         parent::__construct();
 
diff --git a/lib/DAV/Server.php b/lib/DAV/Server.php
index 1a97951ecd..3133e54ad3 100644
--- a/lib/DAV/Server.php
+++ b/lib/DAV/Server.php
@@ -211,7 +211,7 @@ class Server implements LoggerAwareInterface, EmitterInterface
      *
      * @throws Exception
      */
-    public function __construct($treeOrNode = null, HTTP\Sapi $sapi = null)
+    public function __construct($treeOrNode = null, ?HTTP\Sapi $sapi = null)
     {
         if ($treeOrNode instanceof Tree) {
             $this->tree = $treeOrNode;
@@ -882,7 +882,7 @@ public function getHTTPHeaders($path)
      *
      * @return \Traversable
      */
-    private function generatePathNodes(PropFind $propFind, array $yieldFirst = null)
+    private function generatePathNodes(PropFind $propFind, ?array $yieldFirst = null)
     {
         if (null !== $yieldFirst) {
             yield $yieldFirst;
diff --git a/tests/Sabre/CalDAV/SharedCalendarTest.php b/tests/Sabre/CalDAV/SharedCalendarTest.php
index 7e8d830a26..f5ce9d796c 100644
--- a/tests/Sabre/CalDAV/SharedCalendarTest.php
+++ b/tests/Sabre/CalDAV/SharedCalendarTest.php
@@ -11,7 +11,7 @@ class SharedCalendarTest extends \PHPUnit\Framework\TestCase
 {
     protected $backend;
 
-    public function getInstance(array $props = null)
+    public function getInstance(?array $props = null)
     {
         if (is_null($props)) {
             $props = [
diff --git a/tests/Sabre/DAV/Mock/Collection.php b/tests/Sabre/DAV/Mock/Collection.php
index 3ffb342ddd..7baa6f6216 100644
--- a/tests/Sabre/DAV/Mock/Collection.php
+++ b/tests/Sabre/DAV/Mock/Collection.php
@@ -34,7 +34,7 @@ class Collection extends DAV\Collection
      * @param string     $name
      * @param Collection $parent
      */
-    public function __construct($name, array $children = [], Collection $parent = null)
+    public function __construct($name, array $children = [], ?Collection $parent = null)
     {
         $this->name = $name;
         foreach ($children as $key => $value) {
diff --git a/tests/Sabre/DAV/Mock/File.php b/tests/Sabre/DAV/Mock/File.php
index d48ddaa922..329c23e08c 100644
--- a/tests/Sabre/DAV/Mock/File.php
+++ b/tests/Sabre/DAV/Mock/File.php
@@ -30,7 +30,7 @@ class File extends DAV\File
      * @param Collection $parent
      * @param int        $lastModified
      */
-    public function __construct($name, $contents, Collection $parent = null, $lastModified = -1)
+    public function __construct($name, $contents, ?Collection $parent = null, $lastModified = -1)
     {
         $this->name = $name;
         $this->put($contents);
diff --git a/tests/Sabre/DAVACL/PrincipalBackend/Mock.php b/tests/Sabre/DAVACL/PrincipalBackend/Mock.php
index 359e678197..4a721dd6ba 100644
--- a/tests/Sabre/DAVACL/PrincipalBackend/Mock.php
+++ b/tests/Sabre/DAVACL/PrincipalBackend/Mock.php
@@ -9,7 +9,7 @@ class Mock extends AbstractBackend
     public $groupMembers = [];
     public $principals;
 
-    public function __construct(array $principals = null)
+    public function __construct(?array $principals = null)
     {
         $this->principals = $principals;