Testing Django application with --parallel argument in Shippable

I have an application in django / python. I use Shippable to run my tests.

In the shippable configuration file "shippable.yml", I use the following command:

coverage run --source='.' manage.py test apps/ --parallel 2

But with that, Shippable displays a false coverage at just 23%

But if I remove --parallel 2 like this:

coverage run --source='.' manage.py test apps/ I get 58%

Do you know how can I get the correct coverage using --parallel 2?