diff --git a/addon/initializers/add-modals-container.js b/addon/initializers/add-modals-container.js
index 63fa0e98..8cfff887 100644
--- a/addon/initializers/add-modals-container.js
+++ b/addon/initializers/add-modals-container.js
@@ -39,7 +39,7 @@ export default function(AppOrEngine) {
 
   AppOrEngine.register(
     'config:modals-container-id',
-    Ember.testing ? 'ember-testing' : modalContainerElId,
+    { toString: () => Ember.testing ? 'ember-testing' : modalContainerElId },
     { instantiate: false }
   );