TypeError: getattr(): attribute name must be string in Django

I'm writting a Django blog. I registered two models in Django admin (Category, Post). I wanted to add a new post with some category but I got following error:

Internal Server Error: /admin/blog/post/add/
Traceback (most recent call last):
  File "/home/pecan/env/lib/python3.4/site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/home/pecan/env/lib/python3.4/site-packages/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/pecan/env/lib/python3.4/site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/pecan/env/lib/python3.4/site-packages/django/contrib/admin/options.py", line 551, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File "/home/pecan/env/lib/python3.4/site-packages/django/utils/decorators.py", line 149, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/home/pecan/env/lib/python3.4/site-packages/django/views/decorators/cache.py", line 57, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "/home/pecan/env/lib/python3.4/site-packages/django/contrib/admin/sites.py", line 224, in inner
    return view(request, *args, **kwargs)
  File "/home/pecan/env/lib/python3.4/site-packages/django/contrib/admin/options.py", line 1508, in add_view
    return self.changeform_view(request, None, form_url, extra_context)
  File "/home/pecan/env/lib/python3.4/site-packages/django/utils/decorators.py", line 67, in _wrapper
    return bound_func(*args, **kwargs)
  File "/home/pecan/env/lib/python3.4/site-packages/django/utils/decorators.py", line 149, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/home/pecan/env/lib/python3.4/site-packages/django/utils/decorators.py", line 63, in bound_func
    return func.__get__(self, type(self))(*args2, **kwargs2)
  File "/home/pecan/env/lib/python3.4/site-packages/django/contrib/admin/options.py", line 1408, in changeform_view
    return self._changeform_view(request, object_id, form_url, extra_context)
  File "/home/pecan/env/lib/python3.4/site-packages/django/contrib/admin/options.py", line 1440, in _changeform_view
    if form.is_valid():
  File "/home/pecan/env/lib/python3.4/site-packages/django/forms/forms.py", line 183, in is_valid
    return self.is_bound and not self.errors
  File "/home/pecan/env/lib/python3.4/site-packages/django/forms/forms.py", line 175, in errors
    self.full_clean()
  File "/home/pecan/env/lib/python3.4/site-packages/django/forms/forms.py", line 386, in full_clean
    self._post_clean()
  File "/home/pecan/env/lib/python3.4/site-packages/django/forms/models.py", line 414, in _post_clean
    self.validate_unique()
  File "/home/pecan/env/lib/python3.4/site-packages/django/forms/models.py", line 423, in validate_unique
    self.instance.validate_unique(exclude=exclude)
  File "/home/pecan/env/lib/python3.4/site-packages/django/db/models/base.py", line 1034, in validate_unique
    date_errors = self._perform_date_checks(date_checks)
  File "/home/pecan/env/lib/python3.4/site-packages/django/db/models/base.py", line 1145, in _perform_date_checks
    date = getattr(self, unique_for)
TypeError: getattr(): attribute name must be string
[21/Sep/2017 22:16:01] "POST /admin/blog/post/add/ HTTP/1.1" 500 143101
[21/Sep/2017 22:16:16] "GET /admin/blog/post/add/ HTTP/1.1" 200 7865
[21/Sep/2017 22:16:48] "GET /static/admin/img/calendar-icons.svg HTTP/1.1" 200 1094

What's I am doing wrong?

Here's file models.py:

from django.contrib.auth.admin import User
from django.db import models
from django.utils import timezone


# Create your models here.
class Category(models.Model):
    name = models.CharField(max_length=50)
    description = models.TextField(blank=True)


class Post(models.Model):
    title = models.CharField(max_length=250)
    author = models.ForeignKey(User, on_delete=models.CASCADE, related_name="post_author")
    slug = models.SlugField(unique_for_date=True)
    content = models.TextField(blank=True)
    created = models.DateField(default=timezone.now)
    modified = models.DateField(auto_now_add=True)
    published = models.DateField(auto_now=True)
    category = models.ForeignKey(Category, on_delete=models.CASCADE, related_name="post_category")

and admin.py:

from django.contrib import admin
from . import models

# Register your models here.
admin.site.register(models.Post)
admin.site.register(models.Category)

Hope for help!