Unicode error when converting lead into opportunity in CRM module with spanish localization

In CRM module when converting a lead without an associated partner to an opportunity openerp raises an Unicode exception instead of showing the Spanish equivalent of the following warning string:

   «You must assign a partner to this lead before converting to opportunity. You can use the convert to partner button. .»

Obviously this just happens when using the Spanish language. In English everything is fine

The exception is:

[18/Feb/2010:19:02:04] HTTP Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/CherryPy-3.1.2-py2.5.egg/cherrypy/_cprequest.py", line 606, in respond
    cherrypy.response.body = self.handler()
  File "/usr/lib/python2.5/site-packages/CherryPy-3.1.2-py2.5.egg/cherrypy/_cpdispatch.py", line 25, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/controllers/base.py", line 122, in wrapper
    return fn(*args, **kw)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/tools/expose.py", line 204, in func_wrapper
    res = func(*args, **kw)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/tools/utils.py", line 148, in func_wrapper
    return func(*args, **kw)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/tools/utils.py", line 165, in func_wrapper
    return func(*args, **kw)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/controllers/form.py", line 405, in save
    res = self.button_action(params)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/controllers/form.py", line 508, in button_action
    context=params.context or {})
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/controllers/actions.py", line 325, in execute_by_id
    return execute(res, **data)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/controllers/actions.py", line 274, in execute
    return execute_wizard(action['wiz_name'], **data)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/controllers/actions.py", line 109, in execute_wizard
    return Wizard().create(params)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/controllers/base.py", line 122, in wrapper
    return fn(*args, **kw)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/tools/expose.py", line 204, in func_wrapper
    res = func(*args, **kw)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/controllers/wizard.py", line 166, in create
    return self.execute(params)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/controllers/wizard.py", line 85, in execute
    res = rpc.session.execute('wizard', 'execute', wiz_id, datas, state, ctx)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/rpc.py", line 350, in execute
    return self.gateway.execute(obj, method, *args)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/rpc.py", line 154, in execute
    return self.__execute(obj, method, args)
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/rpc.py", line 143, in __execute
    raise common.error(_('Application Error!'), str(e))
  File "/usr/lib/python2.5/site-packages/openerp_web-5.0.6-py2.5.egg/openerp/common.py", line 72, in error
    raise TinyError(message=msg, title=title or _("Error"))
TinyError: unicode() argument 2 must be string, not None

The list of modules I'm currently using are:

account
account_balance_reporting
account_financial_report
account_payment
account_payment_extension
account_tax_include
base
base_contact
base_iban
base_setup
base_vat
crm
crm_configuration
crm_profiling
document
document_ics
hr
l10n_ES_account_balance_report
l10n_ES_extractos_bancarios
l10n_ES_partner
l10n_ES_partner_data
l10n_ES_partner_seq
l10n_ES_pyme_custom
l10n_ES_pyme_install
l10n_ES_remesas
l10n_ES_toponyms
l10n_chart_ES
label
mrp
partner_spam
point_of_sale
process
product
purchase
report_crm
sale
sale_payment
smtpclient
stock
use_control

The branch versions I'm using are:

addons/

------------------------------------------------------------
revno: 2585
author: GPA(OpenERP)
committer: Jay(Open ERP)<email address hidden>
branch nick: 5.0
timestamp: Tue 2010-02-16 17:58:39 +0530

extra-addons/

------------------------------------------------------------
revno: 4250
committer: mra (Open ERP)<email address hidden>
branch nick: stable_5.0-extra-addons
timestamp: Tue 2010-02-16 12:45:41 +0530

openerp-server/

------------------------------------------------------------
revno: 1972
committer: Jay(Open ERP)<email address hidden>
branch nick: 5.0
timestamp: Tue 2010-02-16 17:07:24 +0530

openerp-web/

------------------------------------------------------------
revno: 2811
committer: sma(Tiny) <email address hidden>
branch nick: 5.0
timestamp: Mon 2010-02-08 15:05:04 +0530

openerp-spain/

------------------------------------------------------------
revno: 105
committer: Jordi Esteve <email address hidden>
branch nick: lp-openerp-spain-5.0
timestamp: Tue 2010-02-09 20:33:03 +0100