diff --git a/actionpack/lib/action_controller/response.rb b/actionpack/lib/action_controller/response.rb
index 27860a6207a35edc12136c312f2b8f3f5e122b45..2daeeb920243bdfb8fab35fb5f71d0952f8313b3 100644
--- a/actionpack/lib/action_controller/response.rb
+++ b/actionpack/lib/action_controller/response.rb
@@ -236,7 +236,7 @@ def set_content_length!
         elsif length = headers['Content-Length']
           headers['Content-Length'] = length.to_s
         elsif !body.respond_to?(:call) && (!status || status.to_s[0..2] != '304')
-          headers["Content-Length"] = body.size.to_s
+          headers["Content-Length"] = (body.respond_to?(:bytesize) ? body.bytesize : body.size).to_s
         end
       end
 
diff --git a/actionpack/test/controller/rack_test.rb b/actionpack/test/controller/rack_test.rb
index e458ab6738c947caad8b25b5c24ad148cd3a7979..36ca205d8e71cffe5b8d1a28bd48cbeb82f9174c 100644
--- a/actionpack/test/controller/rack_test.rb
+++ b/actionpack/test/controller/rack_test.rb
@@ -228,6 +228,21 @@ def test_simple_output
     assert_equal ["Hello, World!"], parts
   end
 
+  def test_utf8_output
+    @response.body = [1090, 1077, 1089, 1090].pack("U*")
+    @response.prepare!
+
+    status, headers, body = @response.to_a
+    assert_equal 200, status
+    assert_equal({
+      "Content-Type" => "text/html; charset=utf-8",
+      "Cache-Control" => "private, max-age=0, must-revalidate",
+      "ETag" => '"ebb5e89e8a94e9dd22abf5d915d112b2"',
+      "Set-Cookie" => [],
+      "Content-Length" => "8"
+    }, headers)
+  end
+
   def test_streaming_block
     @response.body = Proc.new do |response, output|
       5.times { |n| output.write(n) }