diff --git a/addons/account_voucher/account_voucher.py b/addons/account_voucher/account_voucher.py
index a8528aa8d6e..8116df538c4 100644
--- a/addons/account_voucher/account_voucher.py
+++ b/addons/account_voucher/account_voucher.py
@@ -144,7 +144,7 @@ class AccountVoucher(models.Model):
     def unlink(self):
         for voucher in self:
             if voucher.state not in ('draft', 'cancel'):
-                raise Warning(_('Cannot delete voucher(s) which are already opened or paid.'))
+                raise UserError(_('Cannot delete voucher(s) which are already opened or paid.'))
         return super(AccountVoucher, self).unlink()
 
     @api.multi