diff --git a/lib/action.php b/lib/action.php
index fafb2c6fc4..670eb498c1 100644
--- a/lib/action.php
+++ b/lib/action.php
@@ -881,6 +881,7 @@ class Action extends HTMLOutputter // lawsuit
      */
     function handle($argarray=null)
     {
+        header('Vary: Accept-Encoding,Cookie');
         $lm   = $this->lastModified();
         $etag = $this->etag();
         if ($etag) {