diff --git a/src/js/contentscript.js b/src/js/contentscript.js
index bcc16345d..4fa8a34a4 100644
--- a/src/js/contentscript.js
+++ b/src/js/contentscript.js
@@ -1502,7 +1502,7 @@ vAPI.domSurveyor = (function() {
 /******************************************************************************/
 
 vAPI.domIsLoaded = function(ev) {
-    if ( ev ) {
+    if ( ev instanceof Event ) {
         document.removeEventListener('DOMContentLoaded', vAPI.domIsLoaded);
     }
     vAPI.domIsLoaded = null;