1 files changed, 5 insertions, 1 deletions
diff --git a/actionpack/lib/action_dispatch/middleware/stack.rb b/actionpack/lib/action_dispatch/middleware/stack.rb
index c52c8c0e6a..5c5362ce4a 100644
--- a/actionpack/lib/action_dispatch/middleware/stack.rb
+++ b/actionpack/lib/action_dispatch/middleware/stack.rb
@@ -122,7 +122,11 @@ module ActionDispatch
       find_all { |middleware| middleware.active? }
     end
 
-    def build(app)
+    def build(app = nil, &blk)
+      app ||= blk
+
+      raise "MiddlewareStack#build requires an app" unless app
+
       active.reverse.inject(app) { |a, e| e.build(a) }
     end
   end