diff --git a/src/lib/ic/TZOPart.php b/src/lib/ic/TZOPart.php
index 57a2501..32f0594 100644
--- a/src/lib/ic/TZOPart.php
+++ b/src/lib/ic/TZOPart.php
@@ -106,10 +106,11 @@ class TZOPart
 
         // chapter markers
         $html = preg_replace_callback(
-            '/<center>(?:# )?(\(\d+\)|([\w\s?]+))<\/center>/',
+            '/<center>(?:# )?(\((\d+)\)|([\w\s?]+))<\/center>/',
             function ($matches) {
                 $number = $matches[1];
-                return '<p class="tzo-chapter" id="chapter'.$number.'">'.$number.'</p>';
+                $id = isset($matches[2]) && $matches[2] != '' ? 'chapter'.$matches[2] : '';
+                return '<p class="tzo-chapter" id="'.$id.'">'.$number.'</p>';
             },
             $html
         );