Comment 2 for bug 873686

 def lease_fixed_ip(self, context, address):
        """Called by dhcp-bridge when ip is leased."""
        LOG.debug(_('Leased IP |%(address)s|'), locals(), context=context)
        fixed_ip = self.db.fixed_ip_get_by_address(context, address)
        if fixed_ip['instance_id'] is None:
            raise exception.Error(_('IP %s leased that is not associated') %
                                      address)
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I see that this was the exception raised. Can you be more specific regarding this exception to be changed to InstanceNotFoundException.