Error when load by VAT in Group Assignation Wizard

Sometimes, when add a list of VATs in this wizard appears this error:
Client Traceback (most recent call last):
  File "/opt/openerp/server/openerp/addons/web/common/http.py", line 180, in dispatch
    response["result"] = method(controller, self, **self.params)
  File "/opt/openerp/server/openerp/addons/web/controllers/main.py", line 1052, in call_button
    action = self.call_common(req, model, method, args, domain_id, context_id)
  File "/opt/openerp/server/openerp/addons/web/controllers/main.py", line 996, in call_common
    return self._call_kw(req, model, method, args, {})
  File "/opt/openerp/server/openerp/addons/web/controllers/main.py", line 1010, in _call_kw
    return getattr(req.session.model(model), method)(*args, **kwargs)
  File "/opt/openerp/server/openerp/addons/web/common/openerplib/main.py", line 250, in proxy
    args, kw)
  File "/opt/openerp/server/openerp/addons/web/common/openerplib/main.py", line 117, in proxy
    result = self.connector.send(self.service_name, method, *args)
  File "/opt/openerp/server/openerp/addons/web/common/http.py", line 608, in send
    raise xmlrpclib.Fault(openerp.tools.exception_to_unicode(e), formatted_info)

Server Traceback (most recent call last):
  File "/opt/openerp/server/openerp/addons/web/common/http.py", line 593, in send
    return openerp.netsvc.dispatch_rpc(service_name, method, args)
  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 167, in execute_kw
    return self.execute(db, uid, obj, method, *args, **kw or {})
  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/timed_groups/wizards/wizard_create_group_assignations.py", line 74, in load_vat
    [('participant.vat','in',re.split('[\s\,\;]+',obj.load_vat))] + (obj.participation_domain or []) ))
TypeError: can only concatenate list (not "str") to list