Cannot send session cookie - headers already sent PHPUnit / Laravel

I have this strange problem when i call the parent::setUp() on my TestCase class for unit test a class when i run phpunit it throw me this error:

1) MatchRequestRepositoryTest::test_find_requests_by_match_id ErrorException: session_start(): Cannot send session cookie - headers already sent by (output started at /var/www/project.dev/vendor/phpunit/phpunit/PHPUnit/TextUI/TestRunner.php:459)

What can be the problem? Thanks for any help.