diff --git a/cms/templates/container_chromeless.html b/cms/templates/container_chromeless.html
index a233e0dc7d..647a68f322 100644
--- a/cms/templates/container_chromeless.html
+++ b/cms/templates/container_chromeless.html
@@ -215,8 +215,8 @@
         // it will report the height of its contents to the parent window when the
         // document loads, window resizes, or DOM mutates.
         if (window !== window.parent) {
-            var lastHeight = window.parent[0].offsetHeight;
-            var lastWidth = window.parent[0].offsetWidth;
+            var lastHeight = window.offsetHeight;
+            var lastWidth = window.offsetWidth;
             var contentElement = document.getElementById('content');
 
             function dispatchResizeMessage(event) {