invoice_line.quantity = Decimal('12.')
  File "/usr/local/lib/python2.7/dist-packages/proteus/__init__.py", line 132, in __set__
    super(FloatDescriptor, self).__set__(instance, value)
  File "/usr/local/lib/python2.7/dist-packages/proteus/__init__.py", line 82, in __set__
    instance._on_change(self.name)
  File "/usr/local/lib/python2.7/dist-packages/proteus/__init__.py", line 857, in _on_change
    list(to_change), context)
  File "/usr/local/lib/python2.7/dist-packages/proteus/config.py", line 149, in __call__
    result = rpc.result(meth(inst, *args, **kwargs))
  File "/home/marius/workplace/rifas/trytond/trytond/model/model.py", line 533, in on_change_with
    changes[fieldname] = getattr(self, method_name)()
  File "/home/marius/workplace/rifas/trytond/trytond/modules/account_invoice/invoice.py", line 1630, in on_change_with_amount
    return currency.round(amount)
  File "/home/marius/workplace/rifas/trytond/trytond/modules/currency/currency.py", line 200, in round
    return (amount / self.rounding).quantize(Decimal('1.'),
  File "/home/marius/workplace/rifas/trytond/trytond/model/fields/field.py", line 183, in __get__
    return inst.__getattr__(self.name)
  File "/home/marius/workplace/rifas/trytond/trytond/model/modelstorage.py", line 1317, in __getattr__
    read_data = self.read(list(ids), ffields.keys())
  File "/home/marius/workplace/rifas/trytond/trytond/model/modelsql.py", line 495, in read
    cls.raise_user_error('read_error', cls.__name__)
  File "/home/marius/workplace/rifas/trytond/trytond/error.py", line 70, in raise_user_error
    raise UserError(error)
UserError: ('UserError', (u"You try to read records that don't exist anymore.\n(Document type: currency.currency)", ''))