Error in django testing: 'str' object has no attribute '_meta'

I am getting the following error when I try to execute testing (coverage run --source='.' manage.py test project) and the problem is that I don't even know in which file the error is happening. I'm not getting any error while executing the

python manage.py runserver / makemigrations / migrate

commands.

I would really appreciate if someone would help me out pin-poining the source of the error!

Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/commands/test.py", line 23, in run_from_argv
    super().run_from_argv(argv)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/base.py", line 330, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/base.py", line 371, in execute
    output = self.handle(*args, **options)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/commands/test.py", line 53, in handle
    failures = test_runner.run_tests(test_labels)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/test/runner.py", line 695, in run_tests
    old_config = self.setup_databases(aliases=databases)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/test/runner.py", line 616, in setup_databases
    self.parallel, **kwargs
  File "/Users/virtual-env/lib/python3.7/site-packages/django/test/utils.py", line 174, in setup_databases
    serialize=connection.settings_dict['TEST'].get('SERIALIZE', True),
  File "/Users/virtual-env/lib/python3.7/site-packages/django/db/backends/base/creation.py", line 77, in create_test_db
    run_syncdb=True,
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/__init__.py", line 168, in call_command
    return command.execute(*args, **defaults)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/base.py", line 371, in execute
    output = self.handle(*args, **options)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/base.py", line 85, in wrapped
    res = handle_func(*args, **kwargs)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/core/management/commands/migrate.py", line 245, in handle
    fake_initial=fake_initial,
  File "/Users/virtual-env/lib/python3.7/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/db/migrations/executor.py", line 227, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/db/migrations/operations/models.py", line 92, in database_forwards
    schema_editor.create_model(model)
  File "/Users/virtual-env/lib/python3.7/site-packages/django/db/backends/base/schema.py", line 331, in create_model
    if field.remote_field.through._meta.auto_created:
AttributeError: 'str' object has no attribute '_meta'