Error in Django Test using Factory Boy

I use Factory Boy in my Django project. Could you please explain my mistake to me: why am I having the error when I ran 'tests.py' - 'ValueError: "" needs to have a value for field "post" before this many-to-many relationship can be used.'

Here is my code:

# factories


class TagFactory(factory.Factory):
    class Meta:
        model = models.Tag

    name = factory.Sequence(lambda n: 'tag-%s' % n)
    slug = factory.Sequence(lambda n: 'slug-%s' % n)


class CategoryFactory(factory.Factory):
    class Meta:
        model = models.Category

    name = factory.Sequence(lambda n: 'cat-%s' % n)
    slug = factory.Sequence(lambda n: 'cat-slug-%s' % n)


class PostFactory(factory.Factory):
    class Meta:
        model = models.Post

    title = factory.Sequence(lambda n: 'postik-%s' % n)
    text = 'some text'
    slug = factory.Sequence(lambda n: 'post_slug_%s' % n)
    category = factory.SubFactory(CategoryFactory)

    @factory.post_generation
    def tags(self, create, extracted, **kwargs):
        if not create:
            # Simple build, do nothing.
            return
        if extracted:
            for tag in extracted:
                self.tags.add(tag)

tests.py

class PostTests(TestCase):
    """
    display_post view test.
    """
    def setUp(self):
        self.tag = factories.TagFactory()
        self.category = factories.CategoryFactory()
        self.poster = factories.PostFactory.create(tags=('tag-1'))

 ...

models.py

class Category(models.Model):
    """
    Category.
    """

    name = models.CharField(max_length=20)
    slug = models.SlugField(max_length=20)

    def __str__(self):
        return self.name

    def get_absolute_url(self):
        return reverse('categorier', args=[str(self.slug)])


class Tag(models.Model):
    """
    Tag.
    """

    name = models.CharField(max_length=100, unique=True)
    slug = models.SlugField(max_length=100, unique=True)

    def __str__(self):
        return self.name

    def get_absolute_url(self):
        return reverse('tagger', args=[str(self.slug)])

    class Meta:
        verbose_name = _('Tag')


class Post(models.Model):
    """
    Post model.
    """

    title = models.CharField(max_length=150)
    created_date = models.DateTimeField(auto_now_add=True)
    image = models.ImageField(upload_to="pictures/%Y/%m/%d",
                                        blank=True, null=True)
    text = RichTextField(max_length=10000)
    slug = models.SlugField(max_length=150, unique=True)
    tags = models.ManyToManyField(Tag, null=True, blank=True)
    category = models.ForeignKey(Category)

    def __str__(self):
        return self.title

    def get_absolute_url(self):
        return reverse('poster', args=[str(self.slug)])

    class Meta:
        ordering = ['-created_date']
        verbose_name = _('Eco Post')
        verbose_name_plural = _('Eco Posts')