Unique field case insensitive constraint

Question

I am trying to ensure that all tags are unique when converted to lowercase. However, when I run the migrate on the following model I receive the following error:

---

api.Tag: (models.E012) 'constraints' refers to the nonexistent field 'Lower(F(name))'.

---

class Tag(models.Model):
  name = models.CharField(max_length=30)


  class Meta:
    constraints = [
        models.UniqueConstraint(
            fields=[Lower('name')],
            name='unique_name'
        ),
    ]