diff --git a/activemodel/lib/active_model/attribute_methods.rb b/activemodel/lib/active_model/attribute_methods.rb
index 02fcaaccd8..f9d2cb5587 100644
--- a/activemodel/lib/active_model/attribute_methods.rb
+++ b/activemodel/lib/active_model/attribute_methods.rb
@@ -375,6 +375,7 @@ module ActiveModel
           end
 
           mod.module_eval <<-RUBY, __FILE__, __LINE__ + 1
+            # frozen_string_literal: true
             #{defn}
               #{body}
             end