diff --git a/control/HTTP.php b/control/HTTP.php
index 03e776efa84..a6a2074a2c7 100644
--- a/control/HTTP.php
+++ b/control/HTTP.php
@@ -287,6 +287,10 @@ public static function add_cache_headers($body = null) {
 		if(self::$cache_age > 0) {
 			$responseHeaders["Cache-Control"] = "max-age=" . self::$cache_age . ", must-revalidate";
 			$responseHeaders["Pragma"] = "";
+
+			// To do: User-Agent should only be added in situations where you *are* actually varying according to user-agent.
+			$responseHeaders['Vary'] = 'Cookie, X-Forwarded-Protocol, User-Agent';
+
 		} else {
 			$responseHeaders["Cache-Control"] = "no-cache, max-age=0, must-revalidate";
 		}