How to update django model char field max length with postgres and heroku

I would like to update max length of django char field of base model and overwrite it in a concrete model that extends from parent, but when i execute migrate command on heroku bash cli appears this error:

django.db.utils.OperationalError: cannot ALTER TABLE "cms_categoria" because it has pending trigger events

Here i leave the change:

Before

class Entidad(models.Model):
  titulo = models.CharField(verbose_name=_("Título"), max_length=100)
  ...

class Categoria(Entidad):
  ...

************

After

class Entidad(models.Model):
  titulo = models.CharField(verbose_name=_("Título"), max_length=30)

class Categoria(Entidad):
  titulo = models.CharField(max_length=20)
  ...

How should I do the update ? Anybody could help me ? Thanks