diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index 1963f04..515cf61 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -51,7 +51,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           # Should be the higest supported version, so we can use the newest tools
-          php-version: '8.3'
+          php-version: '8.4'
           tools: composer, composer-require-checker, composer-unused, phpcs
           extensions: ctype, dom, filter, pcre, spl, xml
           coverage: none
@@ -143,7 +143,7 @@ jobs:
       fail-fast: false
       matrix:
         operating-system: [ubuntu-latest]
-        php-versions: ['8.1', '8.2', '8.3']
+        php-versions: ['8.1', '8.2', '8.3', '8.4']
 
     steps:
       - name: Setup PHP, with composer and extensions
@@ -183,15 +183,15 @@ jobs:
         run: composer install --no-progress --prefer-dist --optimize-autoloader
 
       - name: Run unit tests with coverage
-        if: ${{ matrix.php-versions == '8.3' }}
+        if: ${{ matrix.php-versions == '8.4' }}
         run: vendor/bin/phpunit
 
       - name: Run unit tests (no coverage)
-        if: ${{ matrix.php-versions != '8.3' }}
+        if: ${{ matrix.php-versions != '8.4' }}
         run: vendor/bin/phpunit --no-coverage
 
       - name: Save coverage data
-        if: ${{ matrix.php-versions == '8.3' }}
+        if: ${{ matrix.php-versions == '8.4' }}
         uses: actions/upload-artifact@v4
         with:
           name: coverage-data
@@ -205,7 +205,7 @@ jobs:
       fail-fast: true
       matrix:
         operating-system: [windows-latest]
-        php-versions: ['8.1', '8.2', '8.3']
+        php-versions: ['8.1', '8.2', '8.3', '8.4']
 
     steps:
       - name: Setup PHP, with composer and extensions
diff --git a/src/XML/env_200106/Body.php b/src/XML/env_200106/Body.php
index 202cb13..b977273 100644
--- a/src/XML/env_200106/Body.php
+++ b/src/XML/env_200106/Body.php
@@ -118,7 +118,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this Body to.
      * @return \DOMElement This Body-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200106/Detail.php b/src/XML/env_200106/Detail.php
index 30bdea1..661cad2 100644
--- a/src/XML/env_200106/Detail.php
+++ b/src/XML/env_200106/Detail.php
@@ -89,7 +89,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this Detail to.
      * @return \DOMElement This Detail-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200106/Envelope.php b/src/XML/env_200106/Envelope.php
index e716a8e..d4a11ae 100644
--- a/src/XML/env_200106/Envelope.php
+++ b/src/XML/env_200106/Envelope.php
@@ -102,7 +102,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this envelope to.
      * @return \DOMElement This Envelope-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200106/Fault.php b/src/XML/env_200106/Fault.php
index 94a3b08..50ad4de 100644
--- a/src/XML/env_200106/Fault.php
+++ b/src/XML/env_200106/Fault.php
@@ -116,7 +116,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this fault to.
      * @return \DOMElement This Fault-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200106/Header.php b/src/XML/env_200106/Header.php
index e3f7f31..19c030e 100644
--- a/src/XML/env_200106/Header.php
+++ b/src/XML/env_200106/Header.php
@@ -79,7 +79,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this header to.
      * @return \DOMElement This Header-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/Body.php b/src/XML/env_200305/Body.php
index 3da063d..4425d2b 100644
--- a/src/XML/env_200305/Body.php
+++ b/src/XML/env_200305/Body.php
@@ -116,7 +116,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this Body to.
      * @return \DOMElement This Body-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/Code.php b/src/XML/env_200305/Code.php
index 1c0d005..6b083c0 100644
--- a/src/XML/env_200305/Code.php
+++ b/src/XML/env_200305/Code.php
@@ -102,7 +102,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this code to.
      * @return \DOMElement This Code-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/Detail.php b/src/XML/env_200305/Detail.php
index c8875cf..ece05c6 100644
--- a/src/XML/env_200305/Detail.php
+++ b/src/XML/env_200305/Detail.php
@@ -79,7 +79,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this Detail to.
      * @return \DOMElement This Detail-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/Envelope.php b/src/XML/env_200305/Envelope.php
index 354fd84..fcea539 100644
--- a/src/XML/env_200305/Envelope.php
+++ b/src/XML/env_200305/Envelope.php
@@ -93,7 +93,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this envelope to.
      * @return \DOMElement This Envelope-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/Fault.php b/src/XML/env_200305/Fault.php
index 5de5c0f..b8dfe51 100644
--- a/src/XML/env_200305/Fault.php
+++ b/src/XML/env_200305/Fault.php
@@ -126,7 +126,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this fault to.
      * @return \DOMElement This Fault-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/Header.php b/src/XML/env_200305/Header.php
index a19bda1..d013745 100644
--- a/src/XML/env_200305/Header.php
+++ b/src/XML/env_200305/Header.php
@@ -79,7 +79,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this header to.
      * @return \DOMElement This Header-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/NotUnderstood.php b/src/XML/env_200305/NotUnderstood.php
index 33514ad..b9e755b 100644
--- a/src/XML/env_200305/NotUnderstood.php
+++ b/src/XML/env_200305/NotUnderstood.php
@@ -110,7 +110,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/Reason.php b/src/XML/env_200305/Reason.php
index 3f0ee8e..f43d33e 100644
--- a/src/XML/env_200305/Reason.php
+++ b/src/XML/env_200305/Reason.php
@@ -46,7 +46,7 @@ public function getText(): array
      * @param \DOMElement|null $parent The element we should append this element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/Subcode.php b/src/XML/env_200305/Subcode.php
index 38a2c18..0ba4a45 100644
--- a/src/XML/env_200305/Subcode.php
+++ b/src/XML/env_200305/Subcode.php
@@ -81,7 +81,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this subcode to.
      * @return \DOMElement This Subcode-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/env_200305/SupportedEnvelope.php b/src/XML/env_200305/SupportedEnvelope.php
index 9ece7a5..bdceb98 100644
--- a/src/XML/env_200305/SupportedEnvelope.php
+++ b/src/XML/env_200305/SupportedEnvelope.php
@@ -61,7 +61,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this SupportedEnvelope to.
      * @return \DOMElement This SupportedEnvelope-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('qname', $this->getQName());
diff --git a/src/XML/env_200305/Upgrade.php b/src/XML/env_200305/Upgrade.php
index 4b3f2f1..636e041 100644
--- a/src/XML/env_200305/Upgrade.php
+++ b/src/XML/env_200305/Upgrade.php
@@ -46,7 +46,7 @@ public function getSupportedEnvelope(): array
      * @param \DOMElement|null $parent The element we should append this element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);