diff --git a/app/models/invoice.rb b/app/models/invoice.rb
index 33d75144c..da19b2692 100644
--- a/app/models/invoice.rb
+++ b/app/models/invoice.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Invoice correspond to a single purchase made by an user. This purchase may
 # include reservation(s) and/or a subscription
 class Invoice < ActiveRecord::Base
diff --git a/app/models/invoice_item.rb b/app/models/invoice_item.rb
index 3d0cb940f..f46d430e8 100644
--- a/app/models/invoice_item.rb
+++ b/app/models/invoice_item.rb
@@ -1,3 +1,6 @@
+# frozen_string_literal: true
+
+# A single line inside an invoice. Can be a subscription or a reservation
 class InvoiceItem < ActiveRecord::Base
   belongs_to :invoice
   belongs_to :subscription