diff --git a/addons/purchase/models/purchase.py b/addons/purchase/models/purchase.py
index 5ff8dcae7ae..72bfab95f50 100644
--- a/addons/purchase/models/purchase.py
+++ b/addons/purchase/models/purchase.py
@@ -598,8 +598,17 @@ class PurchaseOrderLine(models.Model):
         # Reset date, price and quantity since _onchange_quantity will provide default values
         self.date_planned = datetime.today().strftime(DEFAULT_SERVER_DATETIME_FORMAT)
         self.price_unit = self.product_qty = 0.0
-        self.product_uom = self.product_id.uom_po_id or self.product_id.uom_id
 
+        self._product_id_change()
+
+        self._suggest_quantity()
+        self._onchange_quantity()
+
+    def _product_id_change(self):
+        if not self.product_id:
+            return
+
+        self.product_uom = self.product_id.uom_po_id or self.product_id.uom_id
         product_lang = self.product_id.with_context(
             lang=self.partner_id.lang,
             partner_id=self.partner_id.id,
@@ -611,9 +620,6 @@ class PurchaseOrderLine(models.Model):
 
         self._compute_tax_id()
 
-        self._suggest_quantity()
-        self._onchange_quantity()
-
     @api.onchange('product_id')
     def onchange_product_id_warning(self):
         if not self.product_id: