diff --git a/_config.php b/_config.php
index edb49c0..56469e7 100644
--- a/_config.php
+++ b/_config.php
@@ -6,7 +6,7 @@ Director::addRules(10, array(
 ));
 
 // add the extension to pages
-Object::add_extension('SiteTree', 'GoogleSitemapSiteTreeDecorator');
+if (class_exists('SiteTree')) Object::add_extension('SiteTree', 'GoogleSitemapSiteTreeDecorator');
 
 // if you need to add this to DataObjects include the following in
 // your own _config: