Здравствуйте, я получаю ошибку can only concatenate str (not "NoneType") to str

привет всем у меня такая ошибка (можно конкатенировать только str (не "NoneType") в str). мой код таков:

from django.db import models
from django.db import models
from django.db.models.fields import CharField
from datetime import datetime
from django.contrib.auth.models import User

# Create your models here.

# Create your models here
class features(models.Model):
    name = models.CharField(max_length=100)
    details = models.CharField(max_length=500)

class post(models.Model):
    title = models.CharField(max_length=100)
    author = models.ForeignKey(User, on_delete=models.CASCADE, null=True)
    body = models.TextField()
    created_at = models.DateTimeField(default=datetime.now, blank=True)

    def __str__(self):
        return self.title + ' | ' + self.author

Я сделал миграции и все, и на странице администратора все работает, но на странице сообщения вот что ошибка сиас полностью:

TypeError at /admin/myapp/post/
can only concatenate str (not "NoneType") to str
Request Method: GET
Request URL:    http://127.0.0.1:8000/admin/myapp/post/
Django Version: 3.2.9
Exception Type: TypeError
Exception Value:    
can only concatenate str (not "NoneType") to str
Exception Location: E:\Web-dev\python\newapp\myapp\models.py, line 21, in __str__
Python Executable:  E:\web-dev\venva3.9\Scripts\python.exe
Python Version: 3.9.9
Python Path:    
['E:\\Web-dev\\python\\newapp',
 'C:\\Program '
 'Files\\WindowsApps\\PythonSoftwareFoundation.Python.3.9_3.9.2544.0_x64__qbz5n2kfra8p0\\python39.zip',
 'C:\\Program '
 'Files\\WindowsApps\\PythonSoftwareFoundation.Python.3.9_3.9.2544.0_x64__qbz5n2kfra8p0\\DLLs',
 'C:\\Program '
 'Files\\WindowsApps\\PythonSoftwareFoundation.Python.3.9_3.9.2544.0_x64__qbz5n2kfra8p0\\lib',
 'C:\\Users\\Pedro\\AppData\\Local\\Microsoft\\WindowsApps\\PythonSoftwareFoundation.Python.3.9_qbz5n2kfra8p0',
 'E:\\web-dev\\venva3.9',
 'E:\\web-dev\\venva3.9\\lib\\site-packages',
 'E:\\web-dev\\venva3.9\\lib\\site-packages\\win32',
 'E:\\web-dev\\venva3.9\\lib\\site-packages\\win32\\lib',
 'E:\\web-dev\\venva3.9\\lib\\site-packages\\Pythonwin']
Server time:    Thu, 25 Nov 2021 17:32:43 +0000
Error during template rendering
In template E:\web-dev\venva3.9\lib\site-packages\django\contrib\admin\templates\admin\base.html, error at line 53

can only concatenate str (not "NoneType") to str

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