diff --git a/core/misc/collapse.js b/core/misc/collapse.js
index 77eb9ffb557734437046469b135a23465c78e333..62a7c0ef2fdc326403086c9a49ed88548d2c8848 100644
--- a/core/misc/collapse.js
+++ b/core/misc/collapse.js
@@ -29,7 +29,7 @@ var isDetailsSupported = (function (doc) {
   root.appendChild(el);
   diff = el.offsetHeight;
   el.open = true;
-  diff = diff != el.offsetHeight;
+  diff = diff !== el.offsetHeight;
   root.removeChild(el);
   if (fake) {
     root.parentNode.removeChild(root);