diff --git a/activesupport/lib/active_support/core_ext/string/multibyte.rb b/activesupport/lib/active_support/core_ext/string/multibyte.rb
index 075b206d04..07c0d16398 100644
--- a/activesupport/lib/active_support/core_ext/string/multibyte.rb
+++ b/activesupport/lib/active_support/core_ext/string/multibyte.rb
@@ -12,10 +12,12 @@ class String
   # class. If the proxy class doesn't respond to a certain method, it's forwarded to the encapsulated string.
   #
   #   >> "lj".upcase
-  #   => "LJ"
+  #   => "lj"
   #   >> "lj".mb_chars.upcase.to_s
   #   => "LJ"
   #
+  # NOTE: An above example is useful for pre Ruby 2.4. Ruby 2.4 supports Unicode case mappings.
+  #
   # == Method chaining
   #
   # All the methods on the Chars proxy which normally return a string will return a Chars object. This allows