diff --git a/actionpack/lib/action_controller/test_case.rb b/actionpack/lib/action_controller/test_case.rb
index 33c0201951..85f2501d42 100644
--- a/actionpack/lib/action_controller/test_case.rb
+++ b/actionpack/lib/action_controller/test_case.rb
@@ -112,8 +112,9 @@ module ActionController
           end
         end
 
-        set_header "CONTENT_LENGTH", data.length.to_s
-        set_header "rack.input", StringIO.new(data)
+        data_stream = StringIO.new(data)
+        set_header "CONTENT_LENGTH", data_stream.length.to_s
+        set_header "rack.input", data_stream
       end
 
       fetch_header("PATH_INFO") do |k|
diff --git a/actionpack/test/controller/request/test_request_test.rb b/actionpack/test/controller/request/test_request_test.rb
index fa049fbc9f..da6fa1388b 100644
--- a/actionpack/test/controller/request/test_request_test.rb
+++ b/actionpack/test/controller/request/test_request_test.rb
@@ -11,6 +11,16 @@ class ActionController::TestRequestTest < ActionController::TestCase
     assert_equal nil, ActionController::TestSession::DEFAULT_OPTIONS[:myparam]
   end
 
+  def test_content_length_has_bytes_count_value
+    non_ascii_parameters = { data: { content: "Latin + Кириллица" } }
+    @request.set_header "REQUEST_METHOD", "POST"
+    @request.set_header "CONTENT_TYPE", "application/json"
+    @request.assign_parameters(@routes, "test", "create", non_ascii_parameters,
+                               "/test", [:data, :controller, :action])
+    assert_equal(@request.get_header("CONTENT_LENGTH"),
+                 StringIO.new(non_ascii_parameters.to_json).length.to_s)
+  end
+
   ActionDispatch::Session::AbstractStore::DEFAULT_OPTIONS.each_key do |option|
     test "rack default session options #{option} exists in session options and is default" do
       assert_equal(ActionDispatch::Session::AbstractStore::DEFAULT_OPTIONS[option],