fix publication status explanation

newsbox/models.py

@@ -427,7 +427,7 @@ class NewsboxExpiredBase(six.with_metaclass(NewsboxModelBase, models.Model)):
         This property must be overided if you have more complicated publication status management
         than just the status field.
         """
-        if not self.is_draft and not self.is_archived and self.is_expired:
+        if not self.is_draft and not self.is_archived and self.is_expired(self):
             explanation = _('This content "%s" is not published because '
                             'its publication end date is in the past')
             return explanation % self._meta.verbose_name