Why is the author field left blank?

Question

Django

I'm creating a small blog in Django. I made it so that each user could create his own post and when viewing the post, the username was displayed. However, when creating a post through the site, the author field remains empty, while in the admin panel in the post settings it is possible to select the author of the post. What's my mistake?
models.py

class Post(models.Model):
    theme = models.CharField(max_length=100)
    post_text = models.CharField(max_length=4000)
    pub_date = models.DateTimeField(auto_now_add=True)
    author = models.ForeignKey(
        User,
        on_delete=models.CASCADE,
    )

    def get_absolute_url(self):
        return reverse("Blog:detail", kwargs={'pk': self.pk})

    def __str__(self):
        return self.theme

views.py

class IndexView(generic.ListView):
    queryset = Post.objects.all()
    template_name = 'Blog/index.html'
    context_object_name = 'latest_post_list'


class DetailView(generic.DetailView):

    template_name = "Blog/detail_post.html"
    queryset = Post.objects.all()


class AboutView(generic.TemplateView):
    template_name = 'Blog/about.html'


class CreateView(generic.CreateView):
    model = Post
    template_name = 'Blog/create_post.html'
    fields = ['theme',
              'post_text']

detail_post.html

{% extends 'main.html' %}

{% block content %}
    <h1>{{ post.theme }}</h1>
    <li>{{ post.post_text }}</li>
    <li>{{ post.pub_date }}</li>
    <li>Author: {{ post.author }}</li>
{% endblock %}