diff --git a/sale.py b/sale.py
index e1a37cbcd4..f341f6256a 100644
--- a/sale.py
+++ b/sale.py
@@ -298,6 +298,8 @@ class Promotion(
                 continue
             context = self.get_context_formula(line)
             new_price = self.get_unit_price(**context)
+            if new_price < 0:
+                new_price = Decimal(0)
             if line.unit_price >= new_price:
                 line.unit_price = round_price(new_price)
                 line.promotion = self