ImproperlyConfiguredError about app_name when using namespace in include()

I am currently trying out Django. I use the namespace argument in one of my include()s in urls.py. When I run the server and try to browse,
I get this error.

File "C:\Users\User\AppData\Local\Programs\Python\Python36-32\lib\site-packages\django\urls\conf.py", line 39, in include
    'Specifying a namespace in include() without providing an app_name '
django.core.exceptions.ImproperlyConfigured: Specifying a namespace in include() without providing an app_name is not supported. Set the app_name attribute in the included module, or pass a 2-tuple containing the list of patterns and app_name instead.

These are my urls.py files:

#project/urls.py

from django.conf.urls import include, url
from django.contrib import admin

urlpatterns = [
    url(r'^reviews/', include('reviews.urls', namespace='reviews')),
    url(r'^admin/', include(admin.site.urls)),
]

and

#app/urls.py

from django.conf.urls import url

from . import views

urlpatterns = [
    # ex: /
    url(r'^$', views.review_list, name='review_list'),
    # ex: /review/5/
    url(r'^review/(?P<review_id>[0-9]+)/$', views.review_detail, name='review_detail'),
    # ex: /wine/
    url(r'^wine$', views.wine_list, name='wine_list'),
    # ex: /wine/5/
    url(r'^wine/(?P<wine_id>[0-9]+)/$', views.wine_detail, name='wine_detail'),
]

What do I pass the app_name as stated in the error message?