validation_failed in transaction

[Malolillo]

I try this:
`
card_id = self.tarjeta_BTC

        api = Uphold()
        api.auth_pat(self.PAT)

        txn_id = api.prepare_txn(card_id, self.tarjeta_origen, cantidad, 'BTC')
        transaccion = api.execute_txn(card_id, txn_id)`

and the api.execute_txn returns me this:
{u'code': u'validation_failed', u'errors': {u'object': [{u'message': u'This value is required', u'code': u'required'}]}}

the txn_id is correct, return me the transaccion id: 371cxxxxx-xxxxx-4axx-abxx-xxxxxxxxxx