diff --git a/reference/idn/setup.xml b/reference/idn/setup.xml
index 1d09c39780..4a541c739b 100644
--- a/reference/idn/setup.xml
+++ b/reference/idn/setup.xml
@@ -9,6 +9,9 @@
   &reftitle.required;
   <para>
    The GNU <link xlink:href="&url.idn;">IDN library</link> must be installed.
+   This extension is not compatible with <link
+   linkend="book.intl">Internationalization extension</link> which is part of
+   PHP 5.3.0.
   </para>
  </section>
  <!-- }}} -->