diff --git a/purchase.py b/purchase.py
index ac2477aa7c..de74312319 100644
--- a/purchase.py
+++ b/purchase.py
@@ -1291,11 +1291,12 @@ class PurchaseLine(ModelSQL, ModelView):
             if old_invoice_line.id not in skip_ids:
                 quantity -= Uom.compute_qty(old_invoice_line.unit,
                         old_invoice_line.quantity, self.unit)
-        invoice_line.quantity = quantity
 
         rounding = self.unit.rounding if self.unit else 0
-        if invoice_line.quantity < rounding:
+        invoice_line.quantity = Uom.round(quantity, rounding)
+        if invoice_line.quantity <= 0:
             return []
+
         invoice_line.unit = self.unit
         invoice_line.product = self.product
         invoice_line.unit_price = self.unit_price
@@ -1349,8 +1350,11 @@ class PurchaseLine(ModelSQL, ModelView):
             if move not in skip:
                 quantity -= Uom.compute_qty(move.uom, move.quantity,
                     self.unit)
-        if quantity < self.unit.rounding:
+
+        quantity = Uom.round(quantity, self.unit.rounding)
+        if quantity <= 0:
             return
+
         if not self.purchase.party.supplier_location:
             self.raise_user_error('supplier_location_required', {
                     'purchase': self.purchase.rec_name,