diff --git a/addons/purchase/purchase.py b/addons/purchase/purchase.py
index 1befe7c693d..f3e4af92f3a 100644
--- a/addons/purchase/purchase.py
+++ b/addons/purchase/purchase.py
@@ -596,7 +596,7 @@ class purchase_order(osv.osv):
                 .signal_button_cancel(cr, uid, map(attrgetter('id'), purchase.picking_ids))
             for inv in purchase.invoice_ids:
                 if inv and inv.state not in ('cancel','draft'):
-                    raise osv.except_osv(
+                    raise osv.except_osv(_('Unable to cancel this purchase order.'),
                         _('You must first cancel all invoices related to this purchase order.'))
             self.pool.get('account.invoice') \
                 .signal_invoice_cancel(cr, uid, map(attrgetter('id'), purchase.invoice_ids))