diff --git a/support/tail.js b/support/tail.js
index 018add8d0c..84be14350c 100644
--- a/support/tail.js
+++ b/support/tail.js
@@ -1,5 +1,5 @@
 // The global object is "self" in Web Workers.
-global = (function() { return this; })();
+var global = (function() { return this; })();
 
 /**
  * Save timer references to avoid Sinon interfering (see GH-237).