diff --git a/modules/purchase_trade/sale.py b/modules/purchase_trade/sale.py
index e70b6db..1ec5447 100755
--- a/modules/purchase_trade/sale.py
+++ b/modules/purchase_trade/sale.py
@@ -1583,6 +1583,8 @@ class SaleLine(metaclass=PoolMeta):
         for line in lines:
             if line.id not in old_values:
                 continue
+            if old_values[line.id] is None:
+                continue
             old = Decimal(old_values[line.id] or 0)
             new = Decimal(line.quantity_theorical or 0)
             delta = new - old