diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index f2229d61fb..73a833a23b 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   `url_for` does not modify its arguments when generating polymorphic URLs.
+
+    *Bernerd Schaefer*
+
 *   Make it easier to opt in to `config.force_ssl` and `config.ssl_options` by
     making them less dangerous to try and easier to disable.
 
diff --git a/actionpack/lib/action_dispatch/routing/url_for.rb b/actionpack/lib/action_dispatch/routing/url_for.rb
index 967bbd62f8..883cd9c2c3 100644
--- a/actionpack/lib/action_dispatch/routing/url_for.rb
+++ b/actionpack/lib/action_dispatch/routing/url_for.rb
@@ -180,7 +180,8 @@ module ActionDispatch
         when Symbol
           HelperMethodBuilder.url.handle_string_call self, options
         when Array
-          polymorphic_url(options, options.extract_options!)
+          components = options.dup
+          polymorphic_url(components, components.extract_options!)
         when Class
           HelperMethodBuilder.url.handle_class_call self, options
         else
diff --git a/actionpack/test/controller/url_for_test.rb b/actionpack/test/controller/url_for_test.rb
index 31677f202d..78e883f134 100644
--- a/actionpack/test/controller/url_for_test.rb
+++ b/actionpack/test/controller/url_for_test.rb
@@ -451,6 +451,26 @@ module AbstractController
         end
       end
 
+      def test_url_for_with_array_is_unmodified
+        with_routing do |set|
+          set.draw do
+            namespace :admin do
+              resources :posts
+            end
+          end
+
+          kls = Class.new { include set.url_helpers }
+          kls.default_url_options[:host] = 'www.basecamphq.com'
+
+          original_components = [:new, :admin, :post, { param: 'value' }]
+          components = original_components.dup
+
+          kls.new.url_for(components)
+
+          assert_equal(original_components, components)
+        end
+      end
+
       private
         def extract_params(url)
           url.split('?', 2).last.split('&').sort
diff --git a/actionview/CHANGELOG.md b/actionview/CHANGELOG.md
index f97ca88b87..9ea250643a 100644
--- a/actionview/CHANGELOG.md
+++ b/actionview/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   `url_for` does not modify its arguments when generating polymorphic URLs.
+
+    *Bernerd Schaefer*
+
 *   `number_to_currency` and `number_with_delimiter` now accept custom `delimiter_pattern` option 
      to handle placement of delimiter, to support currency formats like INR 
      
diff --git a/actionview/lib/action_view/routing_url_for.rb b/actionview/lib/action_view/routing_url_for.rb
index 20d6b9a64c..b4cbc80bd5 100644
--- a/actionview/lib/action_view/routing_url_for.rb
+++ b/actionview/lib/action_view/routing_url_for.rb
@@ -105,10 +105,11 @@ module ActionView
       when :back
         _back_url
       when Array
+        components = options.dup
         if _generate_paths_by_default
-          polymorphic_path(options, options.extract_options!)
+          polymorphic_path(components, components.extract_options!)
         else
-          polymorphic_url(options, options.extract_options!)
+          polymorphic_url(components, components.extract_options!)
         end
       else
         method = _generate_paths_by_default ? :path : :url
diff --git a/actionview/test/template/url_helper_test.rb b/actionview/test/template/url_helper_test.rb
index 027b72b354..50b7865f88 100644
--- a/actionview/test/template/url_helper_test.rb
+++ b/actionview/test/template/url_helper_test.rb
@@ -785,6 +785,13 @@ class SessionsController < ActionController::Base
     @session = Session.new(params[:id])
     render inline: "<%= url_for([@workshop, @session]) %>\n<%= link_to('Session', [@workshop, @session]) %>"
   end
+
+  def edit
+    @workshop = Workshop.new(params[:workshop_id])
+    @session = Session.new(params[:id])
+    @url = [@workshop, @session, format: params[:format]]
+    render inline: "<%= url_for(@url) %>\n<%= link_to('Session', @url) %>"
+  end
 end
 
 class PolymorphicControllerTest < ActionController::TestCase
@@ -815,4 +822,11 @@ class PolymorphicControllerTest < ActionController::TestCase
     get :show, params: { workshop_id: 1, id: 1 }
     assert_equal %{/workshops/1/sessions/1\n<a href="/workshops/1/sessions/1">Session</a>}, @response.body
   end
+
+  def test_existing_nested_resource_with_params
+    @controller = SessionsController.new
+
+    get :edit, params: { workshop_id: 1, id: 1, format: "json"  }
+    assert_equal %{/workshops/1/sessions/1.json\n<a href="/workshops/1/sessions/1.json">Session</a>}, @response.body
+  end
 end