Joël Pittet
--- a/src/TocBuilder.php
+++ b/src/TocBuilder.php
        '#item' => NULL,
      ];
+
      $fragment_node = $dom->createDocumentFragment();
      $html = Html::normalize($this->renderer->render($build));
-      $fragment_node->appendXML($html);
+      // Trim to ensure the first child is a DOMElement.
+      $html = trim($html);
+      // Build a new DOMDocument for the HTML.
+      $fragment_dom = Html::load($html);
+      // Get the body element of the new document.
+      $fragment_body = $fragment_dom->getElementsByTagName('body')->item(0);
+      // Import the new (header) node onto the original DOMDocument.
+      $fragment_import_node = $dom->importNode($fragment_body->firstChild, true);
+      $fragment_node->appendChild($fragment_import_node);
+