テストでエラーが出る

python manage.py test polls　とコマンドを打つとエラーが出ました。
https://docs.djangoproject.com/ja/2.0/intro/tutorial04/
をみながら勉強しています。

Tracebackには

======================================================================
ERROR: test_future_question (polls.tests.QuestionDetailViewTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/XXX/Desktop/django-master/mysite/polls/tests.py", line 220, in test_future_question
    url = reverse('polls:detail', args=(future_question.id,))
  File "/Users/XXX/anaconda/envs/py36/lib/python3.6/site-packages/django/urls/base.py", line 91, in reverse
    return force_text(iri_to_uri(resolver._reverse_with_prefix(view, prefix, *args, **kwargs)))
  File "/Users/XXX/anaconda/envs/py36/lib/python3.6/site-packages/django/urls/resolvers.py", line 497, in _rx
    raise NoReverseMatch(msg)
django.urls.exceptions.NoReverseMatch: Reverse for 'detail' with arguments '(1,)' not found. 1 pattern(s) tried: ['p]
　　　　　　　　　　　　　　　　　　　　　　　・
======================================================================
ERROR: test_two_past_questions (polls.tests.QuestionIndexViewTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/XXX/Desktop/django-master/mysite/polls/tests.py", line 206, in test_two_past_questions
    response = self.client.get(reverse('polls:index'))
  File "/Users/XXX/anaconda/envs/py36/lib/python3.6/site-packages/django/test/client.py", line 536, in get
    **extra)
  File "/Users/XXX/anaconda/envs/py36/lib/python3.6/site-packages/django/test/client.py", line 340, in get
    return self.generic('GET', path, secure=secure, **r)
 　　　　　　　　　　　　　　　　　　　　　　・
  File "/Users/XXX/anaconda/envs/py36/lib/python3.6/site-packages/django/template/defaulttags.py", line 458,r
    url = reverse(view_name, args=args, kwargs=kwargs, current_app=current_app)
  File "/Users/XXX/anaconda/envs/py36/lib/python3.6/site-packages/django/urls/base.py", line 91, in reverse
    return force_text(iri_to_uri(resolver._reverse_with_prefix(view, prefix, *args, **kwargs)))
  File "/Users/XXX/anaconda/envs/py36/lib/python3.6/site-packages/django/urls/resolvers.py", line 497, in _rx
    raise NoReverseMatch(msg)
django.urls.exceptions.NoReverseMatch: Reverse for 'detail' with arguments '(2,)' not found. 1 pattern(s) tried: ['p]

----------------------------------------------------------------------

と出ています。
tests.pyには

from django.urls import reverse
import datetime

from django.utils import timezone
from django.test import TestCase

from .models import Question


class QuestionModelTests(TestCase):

    def test_was_published_recently_with_future_question(self):
        """
        was_published_recently() returns False for questions whose pub_date
        is in the future.
        """
        time = timezone.now() + datetime.timedelta(days=30)
        future_question = Question(pub_date=time)
        self.assertIs(future_question.was_published_recently(), False)

    def test_was_published_recently_with_old_question(self):
        """
        was_published_recently() returns False for questions whose pub_date
        is older than 1 day.
        """
        time = timezone.now() - datetime.timedelta(days=1, seconds=1)
        old_question = Question(pub_date=time)
        self.assertIs(old_question.was_published_recently(), False)

    def test_was_published_recently_with_recent_question(self):
        """
        was_published_recently() returns True for questions whose pub_date
        is within the last day.
        """
        time = timezone.now() - datetime.timedelta(hours=23, minutes=59, seconds=59)
        recent_question = Question(pub_date=time)
        self.assertIs(recent_question.was_published_recently(), True)

def create_question(question_text, days):
    """
    Create a question with the given `question_text` and published the
    given number of `days` offset to now (negative for questions published
    in the past, positive for questions that have yet to be published).
    """
    time = timezone.now() + datetime.timedelta(days=days)
    return Question.objects.create(question_text=question_text, pub_date=time)


class QuestionIndexViewTests(TestCase):
    def test_no_questions(self):
        """
        If no questions exist, an appropriate message is displayed.
        """
        response = self.client.get(reverse('polls:index'))
        self.assertEqual(response.status_code, 200)
        self.assertContains(response, "No polls are available.")
        self.assertQuerysetEqual(response.context['latest_question_list'], [])

    def test_past_question(self):
        """
        Questions with a pub_date in the past are displayed on the
        index page.
        """
        create_question(question_text="Past question.", days=-30)
        response = self.client.get(reverse('polls:index'))
        self.assertQuerysetEqual(
            response.context['latest_question_list'],
            ['<Question: Past question.>']
        )

    def test_future_question(self):
        """
        Questions with a pub_date in the future aren't displayed on
        the index page.
        """
        create_question(question_text="Future question.", days=30)
        response = self.client.get(reverse('polls:index'))
        self.assertContains(response, "No polls are available.")
        self.assertQuerysetEqual(response.context['latest_question_list'], [])

    def test_future_question_and_past_question(self):
        """
        Even if both past and future questions exist, only past questions
        are displayed.
        """
        create_question(question_text="Past question.", days=-30)
        create_question(question_text="Future question.", days=30)
        response = self.client.get(reverse('polls:index'))
        self.assertQuerysetEqual(
            response.context['latest_question_list'],
            ['<Question: Past question.>']
        )

    def test_two_past_questions(self):
        """
        The questions index page may display multiple questions.
        """
        create_question(question_text="Past question 1.", days=-30)
        create_question(question_text="Past question 2.", days=-5)
        response = self.client.get(reverse('polls:index'))
        self.assertQuerysetEqual(
            response.context['latest_question_list'],
            ['<Question: Past question 2.>', '<Question: Past question 1.>']
        )


class QuestionDetailViewTests(TestCase):
    def test_future_question(self):
        """
        The detail view of a question with a pub_date in the future
        returns a 404 not found.
        """
        future_question = create_question(question_text='Future question.', days=5)
        url = reverse('polls:detail', args=(future_question.id,))
        response = self.client.get(url)
        self.assertEqual(response.status_code, 404)

    def test_past_question(self):
        """
        The detail view of a question with a pub_date in the past
        displays the question's text.
        """
        past_question = create_question(question_text='Past Question.', days=-5)
        url = reverse('polls:detail', args=(past_question.id,))
        response = self.client.get(url)
        self.assertContains(response, past_question.question_text)

と書き、urls.pyには

from django.conf.urls import url
from . import views

urlpatterns = [
    url('', views.IndexView.as_view(), name='index'),
    url('<int:pk>/', views.DetailView.as_view(), name='detail'),
    url('<int:pk>/results/', views.ResultsView.as_view(), name='results'),
    url('<int:question_id>/vote/', views.vote, name='vote'),
]

と書き、models.pyには

from django.db import models
from django.utils import timezone
import datetime


class Question(models.Model):
    question_text = models.CharField(max_length=200)

    pub_date = models.DateTimeField('date published')

    def was_published_recently(self):
        now = timezone.now()
        return now - datetime.timedelta(days=1) <= self.pub_date <= now

class Choice(models.Model):
    question = models.ForeignKey(Question, on_delete=models.CASCADE)

    choice_text = models.CharField(max_length=200)

    votes = models.IntegerField(default=0)

と書きました。チュートリアルの内容をそのままコピペしてきたので何が問題なのかわかりません。なぜテストでエラーが出るのでしょうか？