diff --git a/src/XML/EncryptedElementTrait.php b/src/XML/EncryptedElementTrait.php
index 5c8c9548..48b381f8 100644
--- a/src/XML/EncryptedElementTrait.php
+++ b/src/XML/EncryptedElementTrait.php
@@ -179,7 +179,7 @@ public static function fromXML(DOMElement $xml): static
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $this->encryptedData->toXML($e);
@@ -196,7 +196,7 @@ public function toXML(DOMElement $parent = null): DOMElement
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    abstract public function instantiateParentElement(DOMElement $parent = null): DOMElement;
+    abstract public function instantiateParentElement(?DOMElement $parent = null): DOMElement;
 
 
     /**
diff --git a/src/XML/ds/AbstractKeyInfoType.php b/src/XML/ds/AbstractKeyInfoType.php
index b266045f..2de04d67 100644
--- a/src/XML/ds/AbstractKeyInfoType.php
+++ b/src/XML/ds/AbstractKeyInfoType.php
@@ -100,7 +100,7 @@ public function getInfo(): array
      * @param \DOMElement|null $parent The element we should append this KeyInfo 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/ds/CanonicalizationMethod.php b/src/XML/ds/CanonicalizationMethod.php
index f7844a3f..ddb319f5 100644
--- a/src/XML/ds/CanonicalizationMethod.php
+++ b/src/XML/ds/CanonicalizationMethod.php
@@ -78,7 +78,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this KeyName element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Algorithm', $this->getAlgorithm());
diff --git a/src/XML/ds/DigestMethod.php b/src/XML/ds/DigestMethod.php
index 9ce95f54..335d5d4e 100644
--- a/src/XML/ds/DigestMethod.php
+++ b/src/XML/ds/DigestMethod.php
@@ -84,7 +84,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this DigestMethod element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Algorithm', $this->getAlgorithm());
diff --git a/src/XML/ds/DsObject.php b/src/XML/ds/DsObject.php
index 72f78f5a..00f0fae0 100644
--- a/src/XML/ds/DsObject.php
+++ b/src/XML/ds/DsObject.php
@@ -123,7 +123,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this ds:Object 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/ds/KeyValue.php b/src/XML/ds/KeyValue.php
index b6f2d018..7677371b 100644
--- a/src/XML/ds/KeyValue.php
+++ b/src/XML/ds/KeyValue.php
@@ -100,7 +100,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this KeyValue 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/ds/Manifest.php b/src/XML/ds/Manifest.php
index b78c53cd..9eac89ab 100644
--- a/src/XML/ds/Manifest.php
+++ b/src/XML/ds/Manifest.php
@@ -88,7 +88,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Manifest 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/ds/RSAKeyValue.php b/src/XML/ds/RSAKeyValue.php
index 1a23e70b..ff56e935 100644
--- a/src/XML/ds/RSAKeyValue.php
+++ b/src/XML/ds/RSAKeyValue.php
@@ -104,7 +104,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this RSAKeyValue 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/ds/Reference.php b/src/XML/ds/Reference.php
index fc7e2e83..86baeab8 100644
--- a/src/XML/ds/Reference.php
+++ b/src/XML/ds/Reference.php
@@ -167,7 +167,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Reference element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         if ($this->getId() !== null) {
diff --git a/src/XML/ds/RetrievalMethod.php b/src/XML/ds/RetrievalMethod.php
index e75fcca8..e14b15c4 100644
--- a/src/XML/ds/RetrievalMethod.php
+++ b/src/XML/ds/RetrievalMethod.php
@@ -100,7 +100,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this RetrievalMethod element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('URI', $this->getURI());
diff --git a/src/XML/ds/Signature.php b/src/XML/ds/Signature.php
index d004a314..93658706 100644
--- a/src/XML/ds/Signature.php
+++ b/src/XML/ds/Signature.php
@@ -161,7 +161,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Signature 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/ds/SignatureMethod.php b/src/XML/ds/SignatureMethod.php
index 095e1ecb..df2470e3 100644
--- a/src/XML/ds/SignatureMethod.php
+++ b/src/XML/ds/SignatureMethod.php
@@ -76,7 +76,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this SignatureMethod element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Algorithm', $this->getAlgorithm());
diff --git a/src/XML/ds/SignatureProperties.php b/src/XML/ds/SignatureProperties.php
index cd5ff196..cb8cbe10 100644
--- a/src/XML/ds/SignatureProperties.php
+++ b/src/XML/ds/SignatureProperties.php
@@ -89,7 +89,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this SignatureProperties 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/ds/SignatureProperty.php b/src/XML/ds/SignatureProperty.php
index c8b312c6..858b16f4 100644
--- a/src/XML/ds/SignatureProperty.php
+++ b/src/XML/ds/SignatureProperty.php
@@ -102,7 +102,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this SignatureProperty element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Target', $this->getTarget());
diff --git a/src/XML/ds/SignatureValue.php b/src/XML/ds/SignatureValue.php
index d023cf4e..d90f3c7d 100644
--- a/src/XML/ds/SignatureValue.php
+++ b/src/XML/ds/SignatureValue.php
@@ -70,7 +70,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this SignatureValue element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->textContent = $this->getContent();
diff --git a/src/XML/ds/SignedInfo.php b/src/XML/ds/SignedInfo.php
index 0779cc42..4c83d61d 100644
--- a/src/XML/ds/SignedInfo.php
+++ b/src/XML/ds/SignedInfo.php
@@ -172,7 +172,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this SignedInfo 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/ds/Transform.php b/src/XML/ds/Transform.php
index 867b4891..b18abb31 100644
--- a/src/XML/ds/Transform.php
+++ b/src/XML/ds/Transform.php
@@ -127,7 +127,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Transform 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/ds/Transforms.php b/src/XML/ds/Transforms.php
index db46c4fe..aef93203 100644
--- a/src/XML/ds/Transforms.php
+++ b/src/XML/ds/Transforms.php
@@ -76,7 +76,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Transforms 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/ds/X509Data.php b/src/XML/ds/X509Data.php
index b9c68947..d810143d 100644
--- a/src/XML/ds/X509Data.php
+++ b/src/XML/ds/X509Data.php
@@ -98,7 +98,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this X509Data 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/ds/X509IssuerSerial.php b/src/XML/ds/X509IssuerSerial.php
index 483d1906..dc0a70da 100644
--- a/src/XML/ds/X509IssuerSerial.php
+++ b/src/XML/ds/X509IssuerSerial.php
@@ -90,7 +90,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this X509IssuerSerial 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/ds/X509SerialNumber.php b/src/XML/ds/X509SerialNumber.php
index d7b65913..a24f19c9 100644
--- a/src/XML/ds/X509SerialNumber.php
+++ b/src/XML/ds/X509SerialNumber.php
@@ -66,7 +66,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this X509SerialNumber element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->textContent = $this->getContent();
diff --git a/src/XML/ds/XPath.php b/src/XML/ds/XPath.php
index cf84778d..087c8b17 100644
--- a/src/XML/ds/XPath.php
+++ b/src/XML/ds/XPath.php
@@ -88,7 +88,7 @@ public static function fromXML(DOMElement $xml): static
      * @param DOMElement|null $parent
      * @return DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->textContent = $this->getExpression();
diff --git a/src/XML/dsig11/KeyInfoReference.php b/src/XML/dsig11/KeyInfoReference.php
index e1bdc5f6..b9a6a78e 100644
--- a/src/XML/dsig11/KeyInfoReference.php
+++ b/src/XML/dsig11/KeyInfoReference.php
@@ -80,7 +80,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this KeyInfoReference element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('URI', $this->getURI());
diff --git a/src/XML/dsig11/X509Digest.php b/src/XML/dsig11/X509Digest.php
index 277dee82..0eda198a 100644
--- a/src/XML/dsig11/X509Digest.php
+++ b/src/XML/dsig11/X509Digest.php
@@ -81,7 +81,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this X509Digest element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->textContent = $this->getContent();
diff --git a/src/XML/ec/InclusiveNamespaces.php b/src/XML/ec/InclusiveNamespaces.php
index 7deff9eb..eb79c964 100644
--- a/src/XML/ec/InclusiveNamespaces.php
+++ b/src/XML/ec/InclusiveNamespaces.php
@@ -68,7 +68,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this InclusiveNamespaces 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/xenc/AbstractAgreementMethodType.php b/src/XML/xenc/AbstractAgreementMethodType.php
index 009887d0..978dbb62 100644
--- a/src/XML/xenc/AbstractAgreementMethodType.php
+++ b/src/XML/xenc/AbstractAgreementMethodType.php
@@ -140,7 +140,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this AgreementMethod to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Algorithm', $this->getAlgorithm());
diff --git a/src/XML/xenc/AbstractDHKeyValueType.php b/src/XML/xenc/AbstractDHKeyValueType.php
index 66b60d81..ff1afde2 100644
--- a/src/XML/xenc/AbstractDHKeyValueType.php
+++ b/src/XML/xenc/AbstractDHKeyValueType.php
@@ -172,7 +172,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this DHKeyValue 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/xenc/AbstractEncryptedType.php b/src/XML/xenc/AbstractEncryptedType.php
index 6a3e7c96..ad005c03 100644
--- a/src/XML/xenc/AbstractEncryptedType.php
+++ b/src/XML/xenc/AbstractEncryptedType.php
@@ -125,7 +125,7 @@ public function getType(): ?string
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/xenc/AbstractEncryptionMethod.php b/src/XML/xenc/AbstractEncryptionMethod.php
index bafc54fa..0b386162 100644
--- a/src/XML/xenc/AbstractEncryptionMethod.php
+++ b/src/XML/xenc/AbstractEncryptionMethod.php
@@ -118,7 +118,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this EncryptionMethod to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Algorithm', $this->getAlgorithm());
diff --git a/src/XML/xenc/AbstractEncryptionPropertiesType.php b/src/XML/xenc/AbstractEncryptionPropertiesType.php
index 63c79f3c..5b9810b8 100644
--- a/src/XML/xenc/AbstractEncryptionPropertiesType.php
+++ b/src/XML/xenc/AbstractEncryptionPropertiesType.php
@@ -75,7 +75,7 @@ public static function fromXML(DOMElement $xml): static
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/xenc/AbstractEncryptionPropertyType.php b/src/XML/xenc/AbstractEncryptionPropertyType.php
index 9a56bc77..a70ff237 100644
--- a/src/XML/xenc/AbstractEncryptionPropertyType.php
+++ b/src/XML/xenc/AbstractEncryptionPropertyType.php
@@ -99,7 +99,7 @@ public static function fromXML(DOMElement $xml): static
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/xenc/AbstractReference.php b/src/XML/xenc/AbstractReference.php
index f67f6e1d..225fdbf1 100644
--- a/src/XML/xenc/AbstractReference.php
+++ b/src/XML/xenc/AbstractReference.php
@@ -74,7 +74,7 @@ public static function fromXML(DOMElement $xml): static
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('URI', $this->getUri());
diff --git a/src/XML/xenc/CipherData.php b/src/XML/xenc/CipherData.php
index 39ddc20a..3da5ac6e 100644
--- a/src/XML/xenc/CipherData.php
+++ b/src/XML/xenc/CipherData.php
@@ -100,7 +100,7 @@ public static function fromXML(DOMElement $xml): static
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/xenc/CipherReference.php b/src/XML/xenc/CipherReference.php
index e2a169f3..592ee0cf 100644
--- a/src/XML/xenc/CipherReference.php
+++ b/src/XML/xenc/CipherReference.php
@@ -68,7 +68,7 @@ public static function fromXML(DOMElement $xml): static
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('URI', $this->getUri());
diff --git a/src/XML/xenc/EncryptedKey.php b/src/XML/xenc/EncryptedKey.php
index 76754a4e..0dcf0603 100644
--- a/src/XML/xenc/EncryptedKey.php
+++ b/src/XML/xenc/EncryptedKey.php
@@ -242,7 +242,7 @@ public static function fromXML(DOMElement $xml): static
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::toXML($parent);
 
diff --git a/src/XML/xenc/KeySize.php b/src/XML/xenc/KeySize.php
index c2faef38..c0864922 100644
--- a/src/XML/xenc/KeySize.php
+++ b/src/XML/xenc/KeySize.php
@@ -60,7 +60,7 @@ 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);
         $e->textContent = strval($this->getKeySize());
diff --git a/src/XML/xenc/ReferenceList.php b/src/XML/xenc/ReferenceList.php
index 1791b75d..537768e8 100644
--- a/src/XML/xenc/ReferenceList.php
+++ b/src/XML/xenc/ReferenceList.php
@@ -98,7 +98,7 @@ public static function fromXML(DOMElement $xml): static
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/xenc/Transforms.php b/src/XML/xenc/Transforms.php
index 02f20449..078f252c 100644
--- a/src/XML/xenc/Transforms.php
+++ b/src/XML/xenc/Transforms.php
@@ -77,7 +77,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Transforms element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);