diff --git a/config/routes/admin.rb b/config/routes/admin.rb
index b6e945c4c36..4573878eded 100644
--- a/config/routes/admin.rb
+++ b/config/routes/admin.rb
@@ -67,7 +67,7 @@ namespace :admin do
     end
   end
 
-  resources :instances, only: [:index, :show, :destroy], constraints: { id: %r{[^/]+} } do
+  resources :instances, only: [:index, :show, :destroy], constraints: { id: %r{[^/]+} }, format: 'html' do
     member do
       post :clear_delivery_errors
       post :restart_delivery