diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 5b9d45d871..a549b28f16 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -371,7 +371,7 @@ module ActiveRecord
       end
 
       def foreign_key
-        @foreign_key ||= options[:foreign_key] || derive_foreign_key
+        @foreign_key ||= options[:foreign_key] || derive_foreign_key.freeze
       end
 
       def association_foreign_key