Update to rails 4.2.0.beta4

app/controllers/application_controller.rb

@@ -36,6 +36,16 @@ private
   def set_locale
     I18n.locale = (params[:locale].presence || I18n.default_locale).to_sym
   end
+  
+  def _render_with_renderer_json(resource, options)
+    serializer = build_json_serializer(resource, options)
+
+    if serializer
+      super(serializer, options)
+    else
+      super
+    end
+  end
 
   def layout_by_resource(*args)
     #if devise_controller?