Performing migrations in Django - django.db.utils.IntegrityError: null values

I am adding the following attribute type choice to my model

class User(AbstractUser):
    GENDER_MALE = 'M'
    GENDER_FEMALE = 'F'
    GENDER_OTHER = 'O'
    GENDER_CHOICES = (
        (GENDER_MALE, u'male'),
        (GENDER_FEMALE, u'female'),
        (GENDER_OTHER, u'other'),
    )
    gender = models.CharField(max_length=1, choices=GENDER_CHOICES, default=GENDER_OTHER,)

When I execute the migration, it tells me the following:

(nrb_dev)➜  project git:(master) ✗ python manage.py makemigrations
Migrations for 'userprofile':
  0010_auto_20160116_2219.py:
    - Alter field gender on user
(nrb_dev)➜  project git:(master) ✗ python manage.py migrate 


Operations to perform:
  Apply all migrations: sessions, userprofile, auth, admin, contenttypes
Running migrations:
  Applying userprofile.0006_auto_20160116_2206...Traceback (most recent call last):
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
psycopg2.IntegrityError: column "gender" contains null values


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "manage.py", line 11, in <module>
    execute_from_command_line(sys.argv)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/core/management/__init__.py", line 350, in execute_from_command_line
    utility.execute()
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/core/management/__init__.py", line 342, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/core/management/base.py", line 348, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/core/management/base.py", line 399, in execute
    output = self.handle(*args, **options)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/core/management/commands/migrate.py", line 200, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/migrations/executor.py", line 92, in migrate
    self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/migrations/executor.py", line 121, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/migrations/executor.py", line 198, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/migrations/migration.py", line 123, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/migrations/operations/fields.py", line 62, in database_forwards
    field,
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 396, in add_field
    self.execute(sql, params)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 110, in execute
    cursor.execute(sql, params)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/backends/utils.py", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/utils.py", line 95, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/home/bgarcial/.virtualenvs/nrb_dev/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
django.db.utils.IntegrityError: column "gender" contains null values

(nrb_dev)➜  project git:(master) ✗

It tells me that the attribute or column gender that I am adding already contains a null value, but I do not understand if a choice type field from a tuple should be specified more when defining it.

I'm following the example as Django recommends but I do not know what my error.

The code of the migration that you are trying to apply is this:

# -*- coding: utf-8 -*-
# Generated by Django 1.9 on 2016-01-16 22:19
from __future__ import unicode_literals

from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('userprofile', '0009_auto_20160116_2214'),
    ]

    operations = [
        migrations.AlterField(
            model_name='user',
            name='gender',
            field=models.CharField(choices=[('M', 'male'), ('F', 'female'), ('O', 'other')], default='O', max_length=1),
        ),
    ]

On the other hand, which is more recommendable, that in a form the user chooses the gender (male and female) using a select box or is it better an item type radio button?