Coverage couldn't run 'manage.py' as Python code

I am using Coverage to measure my test code, but I stuck on a weird problem. I tried the command according to Django Documentation:

coverage run --source='.' manage.py test myapp

But I got this error:

Coverage.py warning: No data was collected. (no-data-collected) Couldn't run 'manage.py' as Python code: SyntaxError: invalid syntax (manage.py, line 16)

It works correctly using python manage.py test myapp.

Here is my manage.py (I didn't modify anything).

#!/usr/bin/env python
"""Django's command-line utility for administrative tasks."""
import os
import sys


def main():
    os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'mytestsite.settings')
    try:
        from django.core.management import execute_from_command_line
    except ImportError as exc:
        raise ImportError(
            "Couldn't import Django. Are you sure it's installed and "
            "available on your PYTHONPATH environment variable? Did you "
            "forget to activate a virtual environment?"
        ) from exc
    execute_from_command_line(sys.argv)


if __name__ == '__main__':
    main()

What am I missing exactly?

I ran my code and coverage in virtualenv, but I don't think this could be a problem.

ENV:

Python 3.6.8
Django 2.2.3
Coverage 4.5.4