Index: src/wp-includes/js/tinymce/plugins/wpeditimage/plugin.js
===================================================================
--- src/wp-includes/js/tinymce/plugins/wpeditimage/plugin.js	(revision 31176)
+++ src/wp-includes/js/tinymce/plugins/wpeditimage/plugin.js	(working copy)
@@ -185,7 +185,7 @@
 		windowWidth = window.innerWidth;
 		toolbarWidth = toolbarNode.offsetWidth;
 		toolbarHalf = toolbarWidth / 2;
-		iframe = editor.getContentAreaContainer().firstChild;
+		iframe = editor.getContentAreaContainer().getElementsByTagName( 'iframe' )[0];
 		iframePos = DOM.getPos( iframe );
 		iframeWidth = iframe.offsetWidth;
 		iframeHeigth = iframe.offsetHeight;