Can't run django tests by tag

I have problem because I wanted to tag some test in django but when I type in console

from django.test import tag, TestCase


@tag('fast', 'something')
class SomeTestCase(TestCase):
    def setUp(self):
        # do something

    def test_something(self):
        # do something

when I type in console:

./manage.py test --tag=fast -k

I got:

Using existing test database for alias 'default'...
System check identified no issues (0 silenced).

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK
Preserving test database for alias 'default'...