diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 317e8886f6..8701806419 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -714,7 +714,7 @@ def define_generate_prefix(app, name)
               def optimize_routes_generation?; false; end
 
               define_method :find_script_name do |options|
-                if options.key? :script_name
+                if options.key?(:script_name) && options[:script_name].present?
                   super(options)
                 else
                   script_namer.call(options)
diff --git a/railties/test/railties/engine_test.rb b/railties/test/railties/engine_test.rb
index 43b8bd3535..81782a1842 100644
--- a/railties/test/railties/engine_test.rb
+++ b/railties/test/railties/engine_test.rb
@@ -1070,6 +1070,73 @@ def index
       assert_equal "ok", last_response.body
     end
 
+    test "nested isolated engines should set correct route module prefix" do
+      app = File.readlines("#{app_path}/config/application.rb")
+      app.insert(6, "require \"bukkits/awesome\"")
+      File.open("#{app_path}/config/application.rb", "r+") do |f|
+        f.puts app
+      end
+
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        module Bukkits
+          class Engine < ::Rails::Engine
+            isolate_namespace Bukkits
+          end
+        end
+      RUBY
+
+      @plugin.write "lib/bukkits/awesome.rb", <<-RUBY
+        module Bukkits
+          module Awesome
+            class Engine < ::Rails::Engine
+              isolate_namespace Bukkits::Awesome
+            end
+          end
+        end
+      RUBY
+
+      app_file "config/routes.rb", <<-RUBY
+        Rails.application.routes.draw do
+          mount Bukkits::Engine, at: "/bukkits"
+        end
+
+        Bukkits::Engine.routes.draw do
+          get "/foo" => "foo#index"
+
+          mount Bukkits::Awesome::Engine, at: "/awesome"
+        end
+
+        Bukkits::Awesome::Engine.routes.draw do
+          get "/bar", as: :bar, to: "bar#index"
+        end
+      RUBY
+
+      @plugin.write "app/controllers/bukkits/foo_controller.rb", <<-RUBY
+        class Bukkits::FooController < ActionController::Base
+          def index
+            render plain: bukkits_awesome.bar_path
+          end
+        end
+      RUBY
+
+      @plugin.write "app/controllers/bukkits/awesome/bar_controller.rb", <<-RUBY
+        class Bukkits::Awesome::BarController < ActionController::Base
+          def index
+            render plain: "ok"
+          end
+        end
+      RUBY
+
+      add_to_config("config.action_dispatch.show_exceptions = :none")
+
+      boot_rails
+
+      get("/bukkits/foo")
+      assert_equal "/bukkits/awesome/bar", last_response.body
+      get("/bukkits/awesome/bar")
+      assert_equal "ok", last_response.body
+    end
+
     test "loading seed data" do
       @plugin.write "db/seeds.rb", <<-RUBY
         Bukkits::Engine.config.bukkits_seeds_loaded = true