diff --git a/history.py b/history.py
index 07ee9e4..f0b1b69 100644
--- a/history.py
+++ b/history.py
@@ -1,5 +1,6 @@
 # The COPYRIGHT file at the top level of this repository contains
 # the full copyright notices and license terms.
+from trytond.pool import Pool
 from trytond.model import ModelSQL, ModelView, fields
 from trytond.pyson import Eval