Django: definindo valor default para campos Foreign Key

model.py

def get_symbol_default():
    """
    Busca pelo ícone padrão do sistema
    :return: retorna o valor default do
    """
    try:
        default_icon = Default.objects.first().get('default_icon')
    except BaseException as error:
        raise BaseException(_('Register a default icon.')) from error
    return default_icon

class ModelName(models.Model):
    (...)
    symbol = models.ForeignKey(
        Icon,
        verbose_name=_('symbol'),
        null=True,
        on_delete=models.SET_NULL,
        default=get_symbol_default,
    )