Django custom AppConfig breaks the project - django

Question

I've got a bare-bones demonstration Django project created with Django 2.2. The structure looks like:
my_project/
├── db.sqlite3
├── my_app
│ ├── admin.py
│ ├── apps.py
│ ├── __init__.py
│ ├── models.py
│ ├── urls.py
│ └── views.py
├── manage.py
├── my_project
│ ├── settings.py
│ ├── urls.py
│ └── wsgi.py
└── requirements.txt
This started as a tiny issue: "my_app" was showing up in the Django admin as "MY_APP" (with an underscore). The Django docs say that you can make this prettier by defining a custom app config that subclasses "django.apps.AppConfig", and that typically this subclass would be defined in (using my example) "my_app/apps.py". In fact, using manage.py startapp my_app even generates this for you:
# my_app/apps.py
from django.apps import AppConfig
class MyAppConfig(AppConfig):
name = 'my_app'
The docs also say to "put the dotted path to that subclass in INSTALLED_APPS". Another approach is to set the default_app_config variable in your app's "__init__.py", but that "New applications should avoid default_app_config. Instead they should require the dotted path to the appropriate AppConfig subclass to be configured explicitly in INSTALLED_APPS."
However, if I do add the path to the custom app config into INSTALLED_APPS like this:
# settings.py
...
INSTALLED_APPS = [
'django.contrib.admin',
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.messages',
'django.contrib.staticfiles',
'my_app',
'my_app.apps.MyAppConfig'
]
...
the project immediately crashes with "LookupError: No installed app with label 'admin'."
The error traces back to the reference to admin.site.urls in the project's root "urls.py" (line 6 below):
from django.contrib import admin
from django.urls import include, path
urlpatterns = [
path('admin/', admin.site.urls),
path('my_app/', include('my_app.urls', namespace='my_app'))
]
In conclusion, adding the path to the "apps" module created by "startapp" -- even without
making any changes to that module -- crashes the project. OTOH if I go against the docs' advice and define default_app_config in "my_app/init.py", that does work.
This seems very basic, but Googling it is turning up nothing, which usually means I'm making some fundamental error that I don't realize. Any ideas, anyone?