diff --git a/app.yaml b/app.yaml
index 78ceace..c2c35c1 100644
--- a/app.yaml
+++ b/app.yaml
@@ -69,6 +69,8 @@ handlers:
     Strict-Transport-Security: max-age=31536000; includeSubDomains
 
 pagespeed:
+  url_blacklist:
+  - https://www.cosmopolite.org/externs/*
   enabled_rewriters:
   - InlineImages
   - CollapseWhitespace
diff --git a/externs/cosmopolite.js b/externs/cosmopolite.js
index 48c8fd4..b6bddcd 100644
--- a/externs/cosmopolite.js
+++ b/externs/cosmopolite.js
@@ -6,6 +6,15 @@
  * @author ian@cosmopolite.org (Ian Gulliver)
  */
 
+
+
+/**
+ * @return {number}
+ */
+String.prototype.hashCode = function() {};
+
+
+
 /**
  * @see https://www.cosmopolite.org/reference#callbacks
  * @typedef {{onConnect: (function()|undefined),