Value too long for type character varying(24)

My test cases getting fail since I switch the test database backend from SQLite to Postgres. I am creating a model instance using mixer and it is throwing the following error.
"Value too long for type character varying(24)"
There is a field where it is showing the error:
code = models.CharField(_('Code'), max_length=24, null=False, blank=False,
db_column='code', default=uuid.uuid4, unique=True)
I am not getting whats wrong with it.
Please help.

No repository file or test is named. Start by reproducing the Mixer model creation with PostgreSQL, then trace the generated value against the Django field definition and database schema; done means the fixture is created successfully without exceeding the declared length.

Written by the indexing model from the issue text.