diff --git a/django/views/generic/dates.py b/django/views/generic/dates.py
index 3da05cf8e5..4380cd5464 100644
--- a/django/views/generic/dates.py
+++ b/django/views/generic/dates.py
@@ -485,10 +485,14 @@ class BaseWeekArchiveView(YearMixin, WeekMixin, BaseDateListView):
 
         date_field = self.get_date_field()
         week_format = self.get_week_format()
-        week_start = {
-            '%W': '1',
-            '%U': '0',
-        }[week_format]
+        week_choices = {'%W': '1', '%U': '0'}
+        try:
+            week_start = week_choices[week_format]
+        except KeyError:
+            raise ValueError('Unknown week format %r. Choices are: %s' % (
+                week_format,
+                ', '.join(sorted(week_choices)),
+            ))
         date = _date_from_string(year, self.get_year_format(),
                                  week_start, '%w',
                                  week, week_format)
diff --git a/tests/generic_views/test_dates.py b/tests/generic_views/test_dates.py
index 2daac10b31..8cd2b2a8be 100644
--- a/tests/generic_views/test_dates.py
+++ b/tests/generic_views/test_dates.py
@@ -528,6 +528,10 @@ class WeekArchiveViewTests(TestDataMixin, TestCase):
         self.assertEqual(res.status_code, 200)
         self.assertEqual(res.context['week'], datetime.date(2008, 9, 29))
 
+    def test_unknown_week_format(self):
+        with self.assertRaisesMessage(ValueError, "Unknown week format '%T'. Choices are: %U, %W"):
+            self.client.get('/dates/books/2008/week/39/unknown_week_format/')
+
     def test_datetime_week_view(self):
         BookSigning.objects.create(event_date=datetime.datetime(2008, 4, 2, 12, 0))
         res = self.client.get('/dates/booksignings/2008/week/13/')
diff --git a/tests/generic_views/urls.py b/tests/generic_views/urls.py
index f2af8ad1a3..8fbcbbc8cb 100644
--- a/tests/generic_views/urls.py
+++ b/tests/generic_views/urls.py
@@ -182,6 +182,10 @@ urlpatterns = [
     path('dates/books/<int:year>/week/<int:week>/paginated/', views.BookWeekArchive.as_view(paginate_by=30)),
     path('dates/books/<int:year>/week/no_week/', views.BookWeekArchive.as_view()),
     path('dates/books/<int:year>/week/<int:week>/monday/', views.BookWeekArchive.as_view(week_format='%W')),
+    path(
+        'dates/books/<int:year>/week/<int:week>/unknown_week_format/',
+        views.BookWeekArchive.as_view(week_format='%T'),
+    ),
     path('dates/booksignings/<int:year>/week/<int:week>/', views.BookSigningWeekArchive.as_view()),
 
     # DayArchiveView