diff --git a/js/bootstrap-collapse.js b/js/bootstrap-collapse.js
index ee36840d2a..8134cc42ff 100644
--- a/js/bootstrap-collapse.js
+++ b/js/bootstrap-collapse.js
@@ -71,7 +71,7 @@
 
       this.$element
         .removeClass('collapse')
-        [dimension](size || '')
+        [dimension](size || 'auto')
         [0].offsetWidth
 
       this.$element.addClass('collapse')