Handling unique constraint for custom field in django admin

Hi I'm new to django and just took my first foray into using a custom field for my model. I have a Char field that I always want to keep in lowercase, so I implemented it as a custom field as follows (learned from another post on Stack Overflow):

from django.db import models
from django.db.models.fields import CharField

class LowercaseCharField(CharField):
    def pre_save(self, model_instance, add):
        current_value = getattr(model_instance, self.attname)
        setattr(model_instance, self.attname, current_value.lower())
        return getattr(model_instance, self.attname)


class Item(models.Model):
    name = LowercaseCharField(max_length=50, unique=True)

    def __str__(self):
        return self.name

      

        
        
        
      

    

I checked this in admin, and indeed, the field record is correctly converted to lowercase before saving. Unfortunately, when I tested the uniqueness constraint, the admin did not handle the integrity of the error gracefully. Instead of getting a clean error as I do if this is an exact match with the result:

I am getting an ugly error page:

How do I configure a custom field so that the unique constraint is "early" enough to trigger a graceful error or otherwise change the admin so that this "later" error is handled more gracefully?

(Note: I am just using sqlite3 for my db at the moment)

UPDATE: In case anyone is interested, here's the modified code that worked for me:

class LowercaseCharField(CharField):
    def get_db_prep_value(self, value, connection, prepared=False):
         return value.lower()