diff --git a/js/ui/windowPreview.js b/js/ui/windowPreview.js
index a27d0586f46d04f9de07e8891e431f05adbef37e..f9922f2806a2ff439f941515d952db5ba09a298f 100644
--- a/js/ui/windowPreview.js
+++ b/js/ui/windowPreview.js
@@ -85,7 +85,10 @@ var WindowPreviewLayout = GObject.registerClass({
 
         const childBox = new Clutter.ActorBox();
 
-        for (const child of container) {
+        const children = container.get_children();
+        const nChildren = children.length;
+        for (let index = 0; index < nChildren; index++) {
+            const child = children[index];
             if (!child.visible)
                 continue;
 
@@ -96,7 +99,9 @@ var WindowPreviewLayout = GObject.registerClass({
                     bufferRect.x - this._boundingBox.x1,
                     bufferRect.y - this._boundingBox.y1);
 
-                const [, , natWidth, natHeight] = child.get_preferred_size();
+                const size = child.get_preferred_size();
+                const natWidth = size[2];
+                const natHeight = size[3];
                 childBox.set_size(natWidth, natHeight);
 
                 childBox.x1 *= scaleX;