lib/action_dispatch/middleware/stack.rb in actionpack-5.2.8.1 vs lib/action_dispatch/middleware/stack.rb in actionpack-6.0.0.beta1

@@ -95,11 +95,11 @@
 
     def use(klass, *args, &block)
       middlewares.push(build_middleware(klass, args, block))
     end
 
-    def build(app = nil, &block)
-      middlewares.freeze.reverse.inject(app || block) { |a, e| e.build(a) }
+    def build(app = Proc.new)
+      middlewares.freeze.reverse.inject(app) { |a, e| e.build(a) }
     end
 
     private
 
       def assert_index(index, where)