diff --git a/purchase.py b/purchase.py
index 41a501c3d3..436f801eee 100644
--- a/purchase.py
+++ b/purchase.py
@@ -1261,7 +1261,7 @@ class PurchaseLine(ModelSQL, ModelView):
                 quantity -= Uom.compute_qty(old_invoice_line.unit,
                         old_invoice_line.quantity, self.unit)
 
-        rounding = self.unit.rounding if self.unit else 0
+        rounding = self.unit.rounding if self.unit else 0.01
         invoice_line.quantity = Uom.round(quantity, rounding)
         if invoice_line.quantity <= 0:
             return []