Error in IWL wizard check seances

Environment Information :
System : Linux-3.2.0-32-generic-pae-i686-with-Ubuntu-12.04-precise
OS Name : posix
Distributor ID: Ubuntu
Description: Ubuntu 12.04.1 LTS
Release: 12.04
Codename: precise
Operating System Release : 3.2.0-32-generic-pae
Operating System Version : #51-Ubuntu SMP Wed Sep 26 21:54:23 UTC 2012
Operating System Architecture : 32bit
Operating System Locale : ca_ES.UTF-8
Python Version : 2.7.3
OpenERP-Client Version : 6.1-20120810-233427
Last revision No. & ID :Bazaar Package not Found !Traceback (most recent call last):
  File "/opt/openerp/server/openerp/service/netrpc_server.py", line 64, in run
    result = netsvc.dispatch_rpc(msg[0], msg[1], msg[2:])
  File "/opt/openerp/server/openerp/netsvc.py", line 360, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
  File "/opt/openerp/server/openerp/service/web_services.py", line 586, in dispatch
    res = fn(db, uid, *params)
  File "/opt/openerp/server/openerp/osv/osv.py", line 121, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/opt/openerp/server/openerp/osv/osv.py", line 176, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/opt/openerp/server/openerp/osv/osv.py", line 164, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/opt/openerp/server/openerp/addons/school_iwl/wizard/school_iwl_check_seances.py", line 46, in check_seances
    iwl_obj.check_seances(cr, uid, [x.id for x in wizard.iwl_ids], context=context)
  File "/opt/openerp/server/openerp/addons/school_iwl/school_iwl.py", line 313, in check_seances
    days_no_work=days_no_work_by_classe[iwl.classe_id.id])
  File "/opt/openerp/server/openerp/addons/school_iwl/util_periodicity.py", line 115, in periodicity_datetimes_str
    for a_datetime in cls.periodicity_datetimes(**vals):
  File "/opt/openerp/server/openerp/addons/school_iwl/util_periodicity.py", line 77, in periodicity_datetimes
    local_timezone = pytz.timezone(vals.get('tz','Europe/Madrid'))
  File "/usr/lib/python2.7/dist-packages/pytz/__init__.py", line 157, in timezone
    if zone.upper() == 'UTC':
AttributeError: 'bool' object has no attribute 'upper'