diff --git a/sale.py b/sale.py
index 24b488d..362305f 100644
--- a/sale.py
+++ b/sale.py
@@ -395,7 +395,7 @@ class SaleLine(metaclass=PoolMeta):
 
     @fields.depends('unit_price', 'quantity', 'taxes')
     def on_change_with_unit_price_w_tax(self, name=None):
-        if not self.quantity or not self.unit_price:
+        if not self.quantity or self.unit_price is not None:
             return
         currency_round = self.currency.round
         if self.taxes: