Django 1.10: Flatpage site migration runtime error

Question

I'm currently beginning a project in Django 1.10, trying to use the included flat pages application. After installing the apps within my (base) settings file:

INSTALLED_APPS = [
'django.contrib.admin',
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.messages',
'django.contrib.staticfiles',
'django.contrib.sites',
'django.contrib.flatpages',
'first_app']

SITE_ID = 1  # for use with 'django.contrib.sites'

Yet when I run python manage.py migrate, the console returns the following error:

RuntimeError: Model class django.contrib.sites.models.Site doesn't declare an
explicit app_label and isn't in an application in INSTALLED_APPS.

Can someone explain to me what is causing this error, when I clearly declared the app in my 'INSTALLED_APPS' list?

Thanks in advance for any advice!

NOTE: It may be worth noting that I'm using multiple settings files, but this code is within the base settings file (the other files change small things such as the DEBUG variable), but i've had no problems running the server or any other errors.