diff --git a/addons/web_editor/static/src/js/transcoder.js b/addons/web_editor/static/src/js/transcoder.js
index ca6f751f954..b0831832dd6 100644
--- a/addons/web_editor/static/src/js/transcoder.js
+++ b/addons/web_editor/static/src/js/transcoder.js
@@ -13,7 +13,7 @@ var getMatchedCSSRules = function (a) {
         var sheets = document.styleSheets;
         for(var i = sheets.length-1; i >= 0 ; i--) {
             var rules;
-            if (sheets[i].rules) {
+            if (sheets[i].hasOwnProperty('rules')) {
                 rules = sheets[i].rules;
             } else {
                 //try...catch because Firefox not able to enumerate document.styleSheets[].cssRules[] for cross-domain stylesheets.