diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index 444be944df..9b5d39faea 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -972,7 +972,7 @@ module ActionMailer
       end
 
       def instrument_name
-        "action_mailer"
+        "action_mailer".freeze
       end
 
       ActiveSupport.run_load_hooks(:action_mailer, self)
diff --git a/actionpack/lib/abstract_controller/caching/fragments.rb b/actionpack/lib/abstract_controller/caching/fragments.rb
index c85b4adba1..13fa2b393d 100644
--- a/actionpack/lib/abstract_controller/caching/fragments.rb
+++ b/actionpack/lib/abstract_controller/caching/fragments.rb
@@ -136,7 +136,7 @@ module AbstractController
 
       def instrument_fragment_cache(name, key) # :nodoc:
         payload = instrument_payload(key)
-        ActiveSupport::Notifications.instrument("#{name}.#{instrument_name}", payload) { yield }
+        ActiveSupport::Notifications.instrument("#{name}.#{instrument_name}".freeze, payload) { yield }
       end
     end
   end
diff --git a/actionpack/lib/action_controller/caching.rb b/actionpack/lib/action_controller/caching.rb
index a9a8508abc..954265ad97 100644
--- a/actionpack/lib/action_controller/caching.rb
+++ b/actionpack/lib/action_controller/caching.rb
@@ -38,7 +38,7 @@ module ActionController
       end
 
       def instrument_name
-        "action_controller"
+        "action_controller".freeze
       end
   end
 end