send_keys does not working in django testing

I want functional testing

functional test process is

1. Create_user
2. log in the website using created user
3. click a button named booksale

In myproject/functional_test/test_booksale.py

from selenium import webdriver
from selenium.webdriver.common.keys import Keys
from django.test import LiveServerTestCase

from users.models import User


class OurClientLogInTest(LiveServerTestCase):

    def setUp(self):
        self.browser = webdriver.Firefox()
        ## create 1 user id
        User.objects.create_user(username='MyTestID', password='123', nickname='NICKNAMETEST', phone='01011111111')


    def tearDown(self):
        self.browser.quit()

    def test_our_client_can_log_in_this_website(self):

        # url connect
        self.browser.get(self.live_server_url)

        # web page header title has 'Korea book'
        self.assertIn('Korea book', self.browser.title)

        # click log_in button in main page
        log_in = self.browser.find_element_by_id('log_in')
        log_in.send_keys(Keys.ENTER)
        self.assertIn('로그인', self.browser.title)

        # insert username, password
        # click enter
        username_box = self.browser.find_element_by_name('username')
        password_box = self.browser.find_element_by_name('password')

        username_box.send_keys('MyTestID')
        password_box.send_keys('123')

        self.browser.find_element_by_id('button').click()

        ##redirect main page
        self.assertIn('Korea book', self.browser.title)

        ## click a button named book_sale
        book_sale = self.browser.find_element_by_id('book_sale')
        book_sale.send_keys(Keys.ENTER)

        from IPython import embed; embed()

I command python manage.py functional_test

it show me error

raceback (most recent call last):
  File "/Users/hanminsoo/.pyenv/versions/3.5.1/lib/python3.5/wsgiref/handlers.py", line 137, in run
self.result = application(self.environ, self.start_response)
  File "/Users/hanminsoo/.pyenv/versions/study_alone/lib/python3.5/site-packages/django/test/testcases.py", line 1198, in __call__
return super(FSFilesHandler, self).__call__(environ, start_response)
  File "/Users/hanminsoo/.pyenv/versions/study_alone/lib/python3.5/site-packages/django/core/handlers/wsgi.py", line 177, in __call__
response = self.get_response(request)
  File "/Users/hanminsoo/.pyenv/versions/study_alone/lib/python3.5/site-packages/django/test/testcases.py", line 1181, in get_response
return self.serve(request)
  File "/Users/hanminsoo/.pyenv/versions/study_alone/lib/python3.5/site-packages/django/test/testcases.py", line 1193, in serve
return serve(request, final_rel_path, document_root=self.get_base_dir())
  File "/Users/hanminsoo/.pyenv/versions/study_alone/lib/python3.5/site-packages/django/views/static.py", line 54, in serve
fullpath = os.path.join(document_root, newpath)
  File "/Users/hanminsoo/.pyenv/versions/3.5.1/lib/python3.5/posixpath.py", line 89, in join
genericpath._check_arg_types('join', a, *p)
  File "/Users/hanminsoo/.pyenv/versions/3.5.1/lib/python3.5/genericpath.py", line 143, in _check_arg_types
(funcname, s.__class__.__name__)) from None
TypeError: join() argument must be str or bytes, not 'NoneType'

when I remove the code

book_sale.send_keys(Keys.ENTER)

the test passed very clearly

I can't not understand why book_sale button doesn't send ENTER KEY..

please some body help me