Syntax Error when TWILIO env vars are empty

[kunle05]

$ cat .env
export DJANGO_DEBUG=True

export TWILIO_ACCOUNT_SID=
export TWILIO_AUTH_TOKEN=
export TWILIO_FROM_NUMBER=
export TWILIO_TO_NUMBER= #only use for test_twilio.py

$ python3 manage.py runserver
Performing system checks...

Unhandled exception in thread started by <function check_errors.<locals>.wrapper at 0x7f568bf99400>
Traceback (most recent call last):
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/utils/autoreload.py", line 225, in wrapper
    fn(*args, **kwargs)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/core/management/commands/runserver.py", line 117, in inner_run
    self.check(display_num_errors=True)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/core/management/base.py", line 379, in check
    include_deployment_checks=include_deployment_checks,
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/core/management/base.py", line 366, in _run_checks
    return checks.run_checks(**kwargs)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/core/checks/registry.py", line 71, in run_checks
    new_errors = check(app_configs=app_configs)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/core/checks/urls.py", line 40, in check_url_namespaces_unique
    all_namespaces = _load_all_namespaces(resolver)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/core/checks/urls.py", line 57, in _load_all_namespaces
    url_patterns = getattr(resolver, 'url_patterns', [])
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/utils/functional.py", line 37, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/urls/resolvers.py", line 533, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/utils/functional.py", line 37, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/urls/resolvers.py", line 526, in urlconf_module
    return import_module(self.urlconf_name)
  File "/usr/lib/python3.5/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/home/ubuntu/courtbot-python/courtbot/urls.py", line 22, in <module>
    path('sms/', include('sms.urls')),
  File "/home/ubuntu/.local/lib/python3.5/site-packages/django/urls/conf.py", line 34, in include
    urlconf_module = import_module(urlconf_module)
  File "/usr/lib/python3.5/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/home/ubuntu/courtbot-python/sms/urls.py", line 4, in <module>
    from . import views
  File "/home/ubuntu/courtbot-python/sms/views.py", line 23
    resp.message(f'Sorry, I couldn\'t find case {case_num}. Please use a'
                                                                        ^
SyntaxError: invalid syntax

[hiroTochigi]

I cannot reproduce the same error.