diff --git a/actionpack/lib/action_controller/test_case.rb b/actionpack/lib/action_controller/test_case.rb
index 9e95f5a1c4..7c28fac045 100644
--- a/actionpack/lib/action_controller/test_case.rb
+++ b/actionpack/lib/action_controller/test_case.rb
@@ -398,9 +398,7 @@ module ActionController
       def paramify_values(hash_or_array_or_value)
         case hash_or_array_or_value
         when Hash
-          hash_or_array_or_value.each do |key, value|
-            hash_or_array_or_value[key] = paramify_values(value)
-          end
+          Hash[hash_or_array_or_value.map{|key, value| [key, paramify_values(value)] }]
         when Array
           hash_or_array_or_value.map {|i| paramify_values(i)}
         when Rack::Test::UploadedFile
@@ -413,7 +411,7 @@ module ActionController
       def process(action, parameters = nil, session = nil, flash = nil, http_method = 'GET')
         # Ensure that numbers and symbols passed as params are converted to
         # proper params, as is the case when engaging rack.
-        paramify_values(parameters)
+        parameters = paramify_values(parameters)
 
         # Sanity check for required instance variables so we can give an
         # understandable error message.
@@ -447,7 +445,7 @@ module ActionController
         @controller.params.merge!(parameters)
         build_request_uri(action, parameters)
         @controller.class.class_eval { include Testing }
-        @controller.recycle! 
+        @controller.recycle!
         @controller.process_with_new_base_test(@request, @response)
         @assigns = @controller.respond_to?(:view_assigns) ? @controller.view_assigns : {}
         @request.session.delete('flash') if @request.session['flash'].blank?
diff --git a/actionpack/test/controller/test_test.rb b/actionpack/test/controller/test_test.rb
index e9f84ef0fb..2b360649b8 100644
--- a/actionpack/test/controller/test_test.rb
+++ b/actionpack/test/controller/test_test.rb
@@ -519,6 +519,12 @@ XML
     )
   end
 
+  def test_params_passing_doesnt_modify_in_place
+    page = {:name => "Page name", :month => 4, :year => 2004, :day => 6}
+    get :test_params, :page => page
+    assert_equal 2004, page[:year]
+  end
+
   def test_id_converted_to_string
     get :test_params, :id => 20, :foo => Object.new
     assert_kind_of String, @request.path_parameters['id']