diff --git a/src/Element/BookCollection.php b/src/Element/BookCollection.php
index b467de5..cb32958 100644
--- a/src/Element/BookCollection.php
+++ b/src/Element/BookCollection.php
@@ -10,9 +10,9 @@ class BookCollection {
 	
 	/** @var Book[]|BookCollection[] */
 	private array $array;
-	private BookCollection $parent;
+	private ?BookCollection $parent;
 	
-	private function __construct (string $name, array $a, BookCollection $parent) {
+	private function __construct (string $name, array $a, ?BookCollection $parent) {
 		$this->name = $name;
 		$this->array = $a;
 		$this->parent = $parent;
@@ -20,12 +20,12 @@ class BookCollection {
 	
 	/**
 	 * @param DOMNode $root
-	 * @param BookCollection $parent
+	 * @param ?BookCollection $parent
 	 * @param bool $isRoot
 	 * @return BookCollection
 	 * @throws Exception
 	 */
-	public static function parse (DOMNode $root, BookCollection $parent, bool $isRoot = false): BookCollection {
+	public static function parse (DOMNode $root, ?BookCollection $parent, bool $isRoot = false): BookCollection {
 		$name = BookCollection::ROOT;
 		if (!$isRoot) {
 			if ($root->hasAttributes()) {
diff --git a/src/Element/BookContent/Chapter.php b/src/Element/BookContent/Chapter.php
index 23389cb..8354b97 100644
--- a/src/Element/BookContent/Chapter.php
+++ b/src/Element/BookContent/Chapter.php
@@ -9,9 +9,9 @@ class Chapter {
 	/** @var Chapter[]|Page[] */
 	private array $childs;
 	
-	private Chapter $parent;
+	private ?Chapter $parent;
 	
-	private function __construct (string $name, array $array, Chapter $parent) {
+	private function __construct (string $name, array $array, ?Chapter $parent) {
 		$this->name = $name;
 		$this->childs = $array;
 		$this->parent = $parent;
@@ -19,11 +19,11 @@ class Chapter {
 	
 	/**
 	 * @param DOMNode $xmlData
-	 * @param Chapter $parent
+	 * @param ?Chapter $parent
 	 * @return Chapter
 	 * @throws Exception
 	 */
-	public static function parse (DOMNode $xmlData, Chapter $parent): Chapter {
+	public static function parse (DOMNode $xmlData, ?Chapter $parent): Chapter {
 		if ($xmlData->hasAttributes()) {
 			$attrName = $xmlData->attributes->getNamedItem("name");
 			if ($attrName == null) throw new Exception("Chapter xml data missing attribute \"name\"");
diff --git a/src/Element/LinkCollection.php b/src/Element/LinkCollection.php
index e43438e..5d6caf8 100644
--- a/src/Element/LinkCollection.php
+++ b/src/Element/LinkCollection.php
@@ -10,9 +10,9 @@ class LinkCollection {
 	
 	/** @var Link[]|LinkCollection[] */
 	private array $array;
-	private LinkCollection $parent;
+	private ?LinkCollection $parent;
 	
-	private function __construct (string $name, array $a, LinkCollection $parent) {
+	private function __construct (string $name, array $a, ?LinkCollection $parent) {
 		$this->name = $name;
 		$this->array = $a;
 		$this->parent = $parent;
@@ -20,12 +20,12 @@ class LinkCollection {
 	
 	/**
 	 * @param DOMNode $root
-	 * @param LinkCollection $parent
+	 * @param ?LinkCollection $parent
 	 * @param bool $isRoot
 	 * @return LinkCollection
 	 * @throws Exception
 	 */
-	public static function parse (DOMNode $root, LinkCollection $parent, bool $isRoot = false): LinkCollection {
+	public static function parse (DOMNode $root, ?LinkCollection $parent, bool $isRoot = false): LinkCollection {
 		$name = LinkCollection::ROOT;
 		if (!$isRoot) {
 			if ($root->hasAttributes()) {