diff --git a/actionpack/lib/action_dispatch/middleware/stack.rb b/actionpack/lib/action_dispatch/middleware/stack.rb
index bbf734f103566444202f33f26fa5100918788342..87e627319e7fe726fccb9ac8b163da30f978b61d 100644
--- a/actionpack/lib/action_dispatch/middleware/stack.rb
+++ b/actionpack/lib/action_dispatch/middleware/stack.rb
@@ -112,9 +112,7 @@ def use(*args, &block)
       middlewares.push(middleware)
     end
 
-    def build(app = nil, &block)
-      app ||= block
-      raise "MiddlewareStack#build requires an app" unless app
+    def build(app = Proc.new)
       middlewares.freeze.reverse.inject(app) { |a, e| e.build(a) }
     end