diff --git a/testing/mochitest/tests/SimpleTest/EventUtils.js b/testing/mochitest/tests/SimpleTest/EventUtils.js
index c2ea7211569..985255129c9 100644
--- a/testing/mochitest/tests/SimpleTest/EventUtils.js
+++ b/testing/mochitest/tests/SimpleTest/EventUtils.js
@@ -160,6 +160,7 @@ function __doEventDispatch(aTarget, aCharCode, aKeyCode, aHasShift) {
  */
 function _parseModifiers(aEvent)
 {
+  const masks = Components.interfaces.nsIDOMNSEvent;
   var mval = 0;
   if (aEvent.shiftKey)
     mval |= masks.SHIFT_MASK;