diff --git a/js/collapse.js b/js/collapse.js
index f8b65b393f..b2678e0a54 100644
--- a/js/collapse.js
+++ b/js/collapse.js
@@ -77,7 +77,7 @@
       [dimension](size || 'auto')
       [0].offsetWidth
 
-    this.$element[size !== null ? 'addClass' : 'removeClass']('collapse')
+    this.$element[size != null ? 'addClass' : 'removeClass']('collapse')
 
     return this
   }