diff --git a/Documentation/Models/Gfdl/gfdl.docbook b/Documentation/Models/Gfdl/gfdl.docbook
index a8fef02..f076362 100644
--- a/Documentation/Models/Gfdl/gfdl.docbook
+++ b/Documentation/Models/Gfdl/gfdl.docbook
@@ -1,3 +1,8 @@
+<?xml version="1.0"?>
+<!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
+    "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"
+    []>
+
 <appendix id="licenses-gfdl">
 
     <title>GNU Free Documentation License</title>