diff --git a/purchase.py b/purchase.py
index e65096bcf4..ac8367cb01 100644
--- a/purchase.py
+++ b/purchase.py
@@ -1022,11 +1022,10 @@ class PurchaseLine(ModelSQL, ModelView):
     def on_change_with_amount(self, vals):
         currency_obj = self.pool.get('currency.currency')
         if vals.get('type') == 'line':
-            if isinstance(vals.get('_parent_purchase.currency'), (int, long)):
+            currency = vals.get('_parent_purchase.currency')
+            if currency and isinstance(currency, (int, long)):
                 currency = currency_obj.browse(
                         vals['_parent_purchase.currency'])
-            else:
-                currency = vals['_parent_purchase.currency']
             amount = Decimal(str(vals.get('quantity') or '0.0')) * \
                     (vals.get('unit_price') or Decimal('0.0'))
             if currency: