I Don't get my mistake

from django.test import TestCase

from .models import Writer

class WriterModelTestCase(TestCase):
    '''Tests for the Writer model'''
    def test_writer_creation(TestCase):
        email = "example@example.com"
        writer = Writer.objects.create(name="example",email = email, bio="example")
        self.assertIn(email, writer.mailto())

from django.db import models


class Article(models.Model):
    headline = models.CharField(max_length=255)
    publish_date = models.DateTimeField()
    content = models.TextField()
    writer = models.ForeignKey('Writer')

    def __str__(self):
        return self.headline


class Writer(models.Model):
    name = models.CharField(max_length=255)
    email = models.EmailField()
    bio = models.TextField()

    def __str__(self):
        return self.name

    def mailto(self):
        return '{} <{}>'.format(self.name, self.email)