diff --git a/src/test/getTestDocument.js b/src/test/getTestDocument.js
index cca89f4520..70a7d10607 100644
--- a/src/test/getTestDocument.js
+++ b/src/test/getTestDocument.js
@@ -16,6 +16,8 @@
  * @providesModule getTestDocument
  */
 
+"use strict";
+
 function getTestDocument(markup) {
   var iframe = document.createElement('iframe');
   iframe.style.display = 'none';