Json creation problem with object

Hi,
I've a json file with plone objects and there is one field of the objects giving me an error:

UnicodeDecodeError('ascii', '{"id":"aluminio-prata", "nome":"ALUM\xc3\x8dNIO PRATA", "num_demaos":0, "rendimento": 0.0, "unidade":"litros", "url":"", "particular":, "profissional":, "unidades":},', 36, 37, 'ordinal not in range(128)') (Also, the following error occurred while attempting to render the standard error message, please see the event log for full details: 'NoneType' object has no attribute 'getMethodAliases')

I already know with field is, is the "title" from title = obj.pretty_title_or_id(), when I remove it from here its ok:

json += '{"id":"' + str(id) + '", "nome":"' + title + '", "num_demaos":' + str(num_demaos) + ', "rendimento": ' + str(rendimento) + ', "unidade":"' + str(unidade) + '", "url":"' + link_produto + '", "particular":' + arr_area_particular + ', "profissional":' + arr_area_profissional + ', "unidades":' + json_qtd + '},'

but when I leave this var I've got this error.

UnicodeDecodeError('ascii', '{"id":"aluminio-prata", "nome":"ALUM\xc3\x8dNIO PRATA", "num_demaos":0, "rendimento": 0.0, "unidade":"litros", "url":"", "particular":, "profissional":, "unidades":},', 36, 37, 'ordinal not in range(128)') (Also, the following error occurred while attempting to render the standard error message, please see the event log for full details: 'NoneType' object has no attribute 'getMethodAliases')