Quiero ampliar el modelo de usuario personalizado en django. Copio el código de pegado del sitio web oficial de django. Cuando quiero migrarlo arroja un error

TypeError: expected string or buffer

modelos.py

education=models.CharField(max_length=13)

from django.db import models
from django.contrib.auth.models import (
    BaseUserManager, AbstractBaseUser
)


class MyUserManager(BaseUserManager):
    def create_user(self, email, date_of_birth, password=None):
        if not email:
            raise ValueError('Users must have an email address')

        user = self.model(
            email=self.normalize_email(email),
            date_of_birth=date_of_birth,
        )

        user.set_password(password)
        user.save(using=self._db)
        return user

    def create_superuser(self, email, date_of_birth, password):
        user = self.create_user(email,
            password=password,
            date_of_birth=date_of_birth
        )
        user.is_admin = True
        user.save(using=self._db)
        return user


class MyUser(AbstractBaseUser):
    email = models.EmailField(
        verbose_name='email address',
        max_length=255,
        unique=True,
    )
    date_of_birth = models.CharField(max_length=20)
    is_active = models.BooleanField(default=True)
    is_admin = models.BooleanField(default=False)

    objects = MyUserManager()

    USERNAME_FIELD = 'email'
    REQUIRED_FIELDS = ['date_of_birth']

    def get_full_name(self):
        return self.email

    def get_short_name(self):
        return self.email

    def __unicode__(self):              # __unicode__ on Python 2
        return self.email

    def has_perm(self, perm, obj=None):
        "Does the user have a specific permission?"
        return True

    def has_module_perms(self, app_label):
        "Does the user have permissions to view the app `app_label`?"
        return True

    @property
    def is_staff(self):
        "Is the user a member of staff?"
        return self.is_admin

admin.py

    from django import forms
    from django.contrib import admin
    from django.contrib.auth.models import Group
    from django.contrib.auth.admin import UserAdmin as BaseUserAdmin
    from django.contrib.auth.forms import ReadOnlyPasswordHashField

    from .models import MyUser


    class UserCreationForm(forms.ModelForm):
        password1 = forms.CharField(label='Password', widget=forms.PasswordInput)
        password2 = forms.CharField(label='Password confirmation', widget=forms.PasswordInput)

        class Meta:
            model = MyUser
            fields = ('email', 'date_of_birth')

        def clean_password2(self):
            password1 = self.cleaned_data.get("password1")
            password2 = self.cleaned_data.get("password2")
            if password1 and password2 and password1 != password2:
                raise forms.ValidationError("Passwords don't match")
            return password2

        def save(self, commit=True):
            user = super(UserCreationForm, self).save(commit=False)
            user.set_password(self.cleaned_data["password1"])
            if commit:
                user.save()
            return user


    class UserChangeForm(forms.ModelForm):
        password = ReadOnlyPasswordHashField()

        class Meta:
            model = MyUser
            fields = ('email', 'password', 'date_of_birth', 'is_active', 'is_admin')

        def clean_password(self):
            return self.initial["password"]


    class UserAdmin(BaseUserAdmin):
        form = UserChangeForm
        add_form = UserCreationForm
.
        list_display = ('email', 'date_of_birth', 'is_admin')
        list_filter = ('is_admin',)
        fieldsets = (
            (None, {'fields': ('email', 'password')}),
            ('Personal info', {'fields': ('date_of_birth',)}),
            ('Permissions', {'fields': ('is_admin',)}),
        )
        add_fieldsets = (
            (None, {


          'classes': ('wide',),
            'fields': ('email', 'date_of_birth', 'password1', 'password2')}
        ),
    )
    search_fields = ('email',)
    ordering = ('email',)
    filter_horizontal = ()

    admin.site.register(MyUser, UserAdmin)
    admin.site.unregister(Group)

Error de rastreo

  Rendering model states... DONE
  Applying app.0002_auto_20160420_0647...Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 330, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 390, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 441, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/commands/migrate.py", line 221, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/executor.py", line 110, in migrate
    self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/executor.py", line 147, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/migration.py", line 115, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/operations/fields.py", line 62, in database_forwards
    field,
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/sqlite3/schema.py", line 176, in add_field
    self._remake_table(model, create_fields=[field])
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/sqlite3/schema.py", line 74, in _remake_table
    self.effective_default(field)
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/base/schema.py", line 207, in effective_default
    default = field.get_db_prep_save(default, self.connection)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 710, in get_db_prep_save
    prepared=False)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 1322, in get_db_prep_value
    value = self.get_prep_value(value)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 1317, in get_prep_value
    return self.to_python(value)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 1274, in to_python
    parsed = parse_date(value)
  File "/usr/local/lib/python2.7/dist-packages/django/utils/dateparse.py", line 60, in parse_date
    match = date_re.match(value)
TypeError: expected string or buffer

Contenido de 0002_auto_20160420_0647.py

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models, migrations


class Migration(migrations.Migration):

    dependencies = [
        ('app', '0001_initial'),
    ]

    operations = [
        migrations.AlterModelOptions(
            name='myuser',
            options={},
        ),
        migrations.AlterModelManagers(
            name='myuser',
            managers=[
            ],
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='contact',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='date_joined',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='education',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='first_name',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='groups',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='is_staff',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='is_superuser',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='last_name',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='name',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='user_permissions',
        ),
        migrations.RemoveField(
            model_name='myuser',
            name='username',
        ),
        migrations.AddField(
            model_name='myuser',
            name='date_of_birth',
            field=models.DateField(default=1),
            preserve_default=False,
        ),
        migrations.AddField(
            model_name='myuser',
            name='is_admin',
            field=models.BooleanField(default=False),
        ),
        migrations.AlterField(
            model_name='myuser',
            name='email',
            field=models.EmailField(unique=True, max_length=255, verbose_name=b'email address'),
        ),
        migrations.AlterField(
            model_name='myuser',
            name='is_active',
            field=models.BooleanField(default=True),
        ),
    ]