diff --git a/lib/oneboxer.rb b/lib/oneboxer.rb
index 69011d06e57..72d697c756a 100644
--- a/lib/oneboxer.rb
+++ b/lib/oneboxer.rb
@@ -260,7 +260,7 @@ module Oneboxer
   end
 
   def self.preserve_fragment_url_hosts
-    ['http://github.com']
+    @preserve_fragment_url_hosts ||= ['http://github.com']
   end
 
   def self.external_onebox(url)