diff --git a/activesupport/test/multibyte_chars_test.rb b/activesupport/test/multibyte_chars_test.rb
index 22954a5590..73f640e8e5 100644
--- a/activesupport/test/multibyte_chars_test.rb
+++ b/activesupport/test/multibyte_chars_test.rb
@@ -8,7 +8,7 @@ class MultibyteCharsTest < ActiveSupport::TestCase
 
   def setup
     @proxy_class = ActiveSupport::Multibyte::Chars
-    @chars = @proxy_class.new UNICODE_STRING
+    @chars = @proxy_class.new UNICODE_STRING.dup
   end
 
   def test_wraps_the_original_string
@@ -50,8 +50,6 @@ class MultibyteCharsTest < ActiveSupport::TestCase
   def test_methods_are_forwarded_to_wrapped_string_for_byte_strings
     original_encoding = BYTE_STRING.encoding
     assert_equal BYTE_STRING.length, BYTE_STRING.mb_chars.length
-  ensure
-    BYTE_STRING.force_encoding(original_encoding)
   end
 
   def test_forwarded_method_with_non_string_result_should_be_returned_vertabim
diff --git a/activesupport/test/multibyte_test_helpers.rb b/activesupport/test/multibyte_test_helpers.rb
index fdbe2f4350..90af2dadd4 100644
--- a/activesupport/test/multibyte_test_helpers.rb
+++ b/activesupport/test/multibyte_test_helpers.rb
@@ -1,9 +1,9 @@
 # encoding: utf-8
 
 module MultibyteTestHelpers
-  UNICODE_STRING = 'こにちわ'
-  ASCII_STRING = 'ohayo'
-  BYTE_STRING = "\270\236\010\210\245".force_encoding("ASCII-8BIT")
+  UNICODE_STRING = 'こにちわ'.freeze
+  ASCII_STRING = 'ohayo'.freeze
+  BYTE_STRING = "\270\236\010\210\245".force_encoding("ASCII-8BIT").freeze
 
   def chars(str)
     ActiveSupport::Multibyte::Chars.new(str)