phpunit - markTestSkipped() で @runInSeparateProcess を使用するとエラーが発生します

Question

 /**
  * Tests Response->redirect
  * @runInSeparateProcess
 */
 public function testRedirect() {

    $this->markTestSkipped();
 }

エラーメッセージ：

There was 1 error:

1) ResponseTest::testRedirect

PHPUnit_Framework_Exception: PHP Fatal error:  Uncaught exception 'PDOException' with message 'You cannot serialize or unserialize PDO instances' in -:44

Stack trace:
0 [internal function]: PDO->__sleep()

1 -(44): serialize(Array)

2 -(332): __phpunit_run_isolated_test()

3 {main}
  thrown in - on line 44

Fatal error: Uncaught exception 'PDOException' with message 'You cannot serialize or unserialize PDO instances' in -:44

Stack trace:
0 [internal function]: PDO->__sleep()

1 -(44): serialize(Array)

2 -(332): __phpunit_run_isolated_test()

3 {main}
  thrown in - on line 44