diff --git a/actionpack/lib/action_dispatch/http/request.rb b/actionpack/lib/action_dispatch/http/request.rb
index d320556b5d77718692e929030414b69b91fccd80..eeded9ffd0ec93781fbf6fce71cb1c6bdb0f9350 100644
--- a/actionpack/lib/action_dispatch/http/request.rb
+++ b/actionpack/lib/action_dispatch/http/request.rb
@@ -39,7 +39,7 @@ class Request < Rack::Request
     ENV_METHODS.each do |env|
       class_eval <<-METHOD, __FILE__, __LINE__ + 1
         def #{env.sub(/^HTTP_/n, '').downcase}  # def accept_charset
-          @env["#{env}"]                        #   @env["HTTP_ACCEPT_CHARSET"]
+          @env["#{env}".freeze]                 #   @env["HTTP_ACCEPT_CHARSET"]
         end                                     # end
       METHOD
     end