diff --git a/lib/utf8.rb b/lib/utf8.rb
index 99334004..15f7bffe 100644
--- a/lib/utf8.rb
+++ b/lib/utf8.rb
@@ -4,7 +4,7 @@ if String.method_defined?(:force_encoding)
 
   class String
     def +@
-      force_encoding('utf-8')
+      dup.force_encoding('utf-8')
     end
   end
 else