diff --git a/tryton/common/datetime_.py b/tryton/common/datetime_.py
index 1b8de7a31a..7dc25d019f 100644
--- a/tryton/common/datetime_.py
+++ b/tryton/common/datetime_.py
@@ -275,7 +275,7 @@ class Time(gtk.ComboBoxEntry):
         self.__time = time
 
     def update_label(self):
-        if not self.__time:
+        if self.__time is None:
             self.__entry.set_text('')
             return