diff --git a/reference/mongo/book.xml b/reference/mongo/book.xml
index c9cddefaa9..065591258f 100644
--- a/reference/mongo/book.xml
+++ b/reference/mongo/book.xml
@@ -5,7 +5,8 @@
  <title>Mongo</title>
  <titleabbrev>Mongo</titleabbrev>
 
- <preface>
+ <preface xml:id="intro.mongo">
+  &reftitle.intro;
   <para>
    This extension allows you to connect to MongoDB through PHP.
   </para>