diff --git a/src/accordion/accordion.js b/src/accordion/accordion.js
index 45e5155dd9..7684995198 100644
--- a/src/accordion/accordion.js
+++ b/src/accordion/accordion.js
@@ -82,7 +82,7 @@ angular.module('ui.bootstrap.accordion', ['ui.bootstrap.collapse'])
       scope.openClass = attrs.openClass || 'panel-open';
       scope.panelClass = attrs.panelClass;
       scope.$watch('isOpen', function(value) {
-        element.toggleClass(scope.openClass, value);
+        element.toggleClass(scope.openClass, Boolean(value));
         if (value) {
           accordionCtrl.closeOthers(scope);
         }