diff --git a/purchase.py b/purchase.py
index 39ea72696e..650682b34f 100644
--- a/purchase.py
+++ b/purchase.py
@@ -1847,7 +1847,7 @@ class Invoice(ModelSQL, ModelView):
         if isinstance(ids, (int, long)):
             ids = [ids]
         cursor.execute('SELECT id FROM purchase_invoices_rel ' \
-                'WHERE invoice IN (' + ','.join(['%s' for x in ids]) + ')',
+                'WHERE invoice IN (' + ','.join(('%s',) * len(ids)) + ')',
                 ids)
         if cursor.fetchone():
             self.raise_user_error(cursor, 'delete_purchase_invoice',