PHPUnit + DbUnit tests causes strange PDOException

I'm learning PHPUnit and found a strange behavior in one of my tests.

I'm running all tests together from command line by passing folder with tests as an argument to PHPUnit:

phpunit examples

Everything went fine until I started testing my database using DbUnit. It seems like my database test has conflict with one of my another tests. I got message:

Time: 1.69 seconds, Memory: 4.50Mb

There was 1 error:

1) BackupStaticPropertyTest::test_PropertyShouldBeEqualToSix
PDOException: You cannot serialize or unserialize PDO instances

C:\Apache24\htdocs\phpunit-examples\vendor\sebastian\global-state\src\Snapshot.php:392
C:\Apache24\htdocs\phpunit-examples\vendor\sebastian\global-state\src\Snapshot.php:162

FAILURES!

My BackupStaticPropertyTest:

class BackupStaticPropertyTest extends PHPUnit_Framework_TestCase
{
    public function test_PropertyShouldBeEqualToFive()
    {
        $this->assertEquals(5, StaticProperty::$prop);
        StaticProperty::$prop = 6;
    }


    /**
     * @backupStaticAttributes enabled
     */
    public function test_PropertyShouldBeEqualToSix()
    {
        $this->assertEquals(6, StaticProperty::$prop);
        StaticProperty::$prop = 7;
    }

    public function test_PropertyShouldBeEqualToSeven()
    {
        $this->assertEquals(StaticProperty::$prop, 6);
    }
}

In this test I'm using @backupStaticAttributes annotation. Looks like it's the reason of error. But how does this test can be connected to my Database test?

My database test:

class CustomDatabaseTest extends DefaultDatabaseTestCase
{
    /**
     * @return PHPUnit_Extensions_Database_DataSet_IDataSet
     */
    protected function getDataSet()
    {
        $compositeDataSet = new PHPUnit_Extensions_Database_DataSet_CompositeDataSet();
        $compositeDataSet->addDataSet($this->createXMLDataSet('examples/Database-Test-Example/users-table.xml'));
        $compositeDataSet->addDataSet($this->createXMLDataSet('examples/Database-Test-Example/messages-table.xml'));
        return $compositeDataSet;
    }

    public function test_RowCountForUserTableShouldBeEqualToTwo()
    {
        $this->assertEquals(2, $this->getConnection()->getRowCount('users'));
    }
}

DefaultDatabaseTestCase:

abstract class DefaultDatabaseTestCase extends PHPUnit_Extensions_Database_TestCase
{
    private static $pdo = null;
    private $conn = null;

    final public function getConnection()
    {
        if ($this->conn === null) {
            if (self::$pdo == null) {
                self::$pdo = new PDO($GLOBALS['DB_DSN'], $GLOBALS['DB_USER'],
                    $GLOBALS['DB_PWD']);
                $this->conn = $this->createDefaultDBConnection(self::$pdo, $GLOBALS['DB_DATABASE']);
            }
        }
        return $this->conn;
    }
}

I tried to use:

/**
* @backupGlobals disabled
* @backupStaticAttributes disabled
*/

annotations above my Database testcase class, but still getting an error. Why do I get PDO exception in test that is not about database testing at all?