Mrinalini Kumari
--- a/src/TocBuilder.php
+++ b/src/TocBuilder.php

      // Build the fragment (header) node.
      $build = [];
-      $header_level = (int) $dom_node->tagName[1];
+      if($dom_node && $dom_node->tagName[1]) {
--- a/src/TocBuilder.php
+++ b/src/TocBuilder.php

      // Build the fragment (header) node.
      $build = [];
-      $header_level = (int) $dom_node->tagName[1];
+      if($dom_node && $dom_node->tagName[1]) {
+        if($dom_node && $dom_node->tagName[1]) {
--- a/src/Toc.php
+++ b/src/Toc.php

    $index_keys = $default_keys;

-    $dom = Html::load($this->source);
+    $tidy = new  \tidy();