diff --git a/tryton/gui/window/view_form/view/list_gtk/widget.py b/tryton/gui/window/view_form/view/list_gtk/widget.py
index 778f407bba..cba9b749fe 100644
--- a/tryton/gui/window/view_form/view/list_gtk/widget.py
+++ b/tryton/gui/window/view_form/view/list_gtk/widget.py
@@ -1053,7 +1053,7 @@ class _ReferenceSelection(Selection):
         return False
 
 
-class ProgressBar(object):
+class ProgressBar(Cell):
     align = 0.5
     orientations = {
         'left_to_right': (Gtk.Orientation.HORIZONTAL, False),