server action copy object

I'm using v6.0.2.

If I go to Administration->Customization->Low level objects->Actions->Server Actions and open a server action, then change the action type to "Copy Object" and then select a value in the drop down list labelled as "Copy Of" I get the following error:

Environment Information :
System : Linux-2.6.32-25-generic-pae-i686-with-Ubuntu-10.04-lucid
OS Name : posix
Distributor ID: Ubuntu
Description: Ubuntu 10.04.1 LTS
Release: 10.04
Codename: lucid
Operating System Release : 2.6.32-25-generic-pae
Operating System Version : #45-Ubuntu SMP Sat Oct 16 21:01:33 UTC 2010
Operating System Architecture : 32bit
Operating System Locale : es_ES.UTF8
Python Version : 2.6.5
OpenERP-Client Version : 6.0.2
Last revision No. & ID :1851 launchpad_translations_on_behalf_of_openerp-20110601054158-x1xtoqa1ephp2nr7
<type 'exceptions.TypeError'>:cannot marshal None unless allow_none is enabled

In the terminal where I executed the client I get the following error:

Environment Information :
System : Linux-2.6.32-25-generic-pae-i686-with-Ubuntu-10.04-lucid
OS Name : posix
Distributor ID: Ubuntu
Description: Ubuntu 10.04.1 LTS
Release: 10.04
Codename: lucid
Operating System Release : 2.6.32-25-generic-pae
Operating System Version : #45-Ubuntu SMP Sat Oct 16 21:01:33 UTC 2010
Operating System Architecture : 32bit
Operating System Locale : es_ES.UTF8
Python Version : 2.6.5
OpenERP-Client Version : 6.0.2
Last revision No. & ID :1851 launchpad_translations_on_behalf_of_openerp-20110601054158-x1xtoqa1ephp2nr7
<type 'exceptions.TypeError'>:cannot marshal None unless allow_none is enabled
Traceback (most recent call last):
  File "/openerp/descargas/6.0.2/client/bin/widget/view/form_gtk/reference.py", line 219, in sig_changed_combo
    (self.get_model(), [0, '']))
  File "/openerp/descargas/6.0.2/client/bin/widget/model/field.py", line 496, in set_client
    self.sig_changed(model)
  File "/openerp/descargas/6.0.2/client/bin/widget/model/field.py", line 64, in sig_changed
    model.on_change(self.attrs['on_change'])
  File "/openerp/descargas/6.0.2/client/bin/widget/model/record.py", line 308, in on_change
    response = getattr(self.rpc, func_name)(ids, *args)
  File "/openerp/descargas/6.0.2/client/bin/rpc.py", line 404, in __call__
    return session.rpc_exec_auth('/object', 'execute', self.object, self.func, *args)
  File "/openerp/descargas/6.0.2/client/bin/rpc.py", line 218, in rpc_exec_auth
    return sock.exec_auth(method, *args)
  File "/openerp/descargas/6.0.2/client/bin/rpc.py", line 123, in exec_auth
    res = self.execute(method, self._uid, self._passwd, *args)
  File "/openerp/descargas/6.0.2/client/bin/rpc.py", line 141, in execute
    result = getattr(self._sock,method)(self._db, *args)
  File "/usr/lib/python2.6/xmlrpclib.py", line 1199, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python2.6/xmlrpclib.py", line 1489, in __request
    verbose=self.__verbose
  File "/usr/lib/python2.6/xmlrpclib.py", line 1253, in request
    return self._parse_response(h.getfile(), sock)
  File "/usr/lib/python2.6/xmlrpclib.py", line 1392, in _parse_response
    return u.close()
  File "/usr/lib/python2.6/xmlrpclib.py", line 838, in close
    raise Fault(**self._stack[0])
xmlrpclib.Fault: <Fault 1: "<type 'exceptions.TypeError'>:cannot marshal None unless allow_none is enabled">