diff --git a/actionpack/lib/action_dispatch/middleware/stack.rb b/actionpack/lib/action_dispatch/middleware/stack.rb
index 24be4fe..6c53d39 100644
--- a/actionpack/lib/action_dispatch/middleware/stack.rb
+++ b/actionpack/lib/action_dispatch/middleware/stack.rb
@@ -85,9 +85,10 @@ module ActionDispatch
     end
 
     def insert(index, *args, &block)
-      index = self.index(index) unless index.is_a?(Integer)
+      i = index.is_a?(Integer) ? index : self.index(index)
+      raise "No such middleware to insert before: #{index.inspect}" unless i
       middleware = Middleware.new(*args, &block)
-      super(index, middleware)
+      super(i, middleware)
     end
 
     alias_method :insert_before, :insert