diff --git a/addon/display/panel.js b/addon/display/panel.js
index a6ac74f0ca4d08c4717d27de2efd88fe4518f0c1..7da34a42da49dfee8a7e6f3b0012e12af13f4e77 100644
--- a/addon/display/panel.js
+++ b/addon/display/panel.js
@@ -114,7 +114,7 @@
   }
 
   function isAtTop(cm, dom) {
-    for (let sibling = dom.nextSibling; sibling; sibling = sibling.nextSibling)
+    for (var sibling = dom.nextSibling; sibling; sibling = sibling.nextSibling)
       if (sibling == cm.getWrapperElement()) return true
     return false
   }