diff --git a/barker/barker/routers/voucher/update.py b/barker/barker/routers/voucher/update.py
index 8e15ea7..a39a086 100644
--- a/barker/barker/routers/voucher/update.py
+++ b/barker/barker/routers/voucher/update.py
@@ -137,7 +137,7 @@ def update_route(
                                 inv.quantity
                                 for ko in item.kots
                                 for inv in ko.inventories
-                                if ko.id and inv.product_id == product.id
+                                if ko.id and inv.product_id == product.product_id
                             )
                             * -1,
                             2,