I'm trying to run the test suite for doctrine\mongo-odm on my local machine. Php crashes with no error messages (when I say crash, I mean crash, the php process terminates) on the is test: `Doctrine\ODM\MongoDB\Test\Functional\FilesTest::testFiles`.

The test has the following content:

```
public function testFiles()
{
    $image = new File();
    $image->setName('Test');
    $image->setFile(__DIR__ . '/file.txt');

    $profile = new Profile();
    $profile->setFirstName('Jon');
    $profile->setLastName('Wage');
    $profile->setImage($image);

    $this->dm->persist($profile);
    $this->dm->flush();

    $this->assertInstanceOf('Doctrine\MongoDB\GridFSFile', $image->getFile());
    $this->assertFalse($image->getFile()->isDirty());
    $this->assertEquals(__DIR__ . '/file.txt', $image->getFile()->getFilename());
    $this->assertTrue(file_exists($image->getFile()->getFilename()));
    $this->assertEquals('These are the bytes...', $image->getFile()->getBytes());

    $image->setName('testing');
    $this->dm->flush();
    $this->dm->clear();

fails here ===>>$image = $this->dm->find('Documents\File', $image->getId());
    $this->assertNotNull($image);
    $this->assertEquals('testing', $image->getName());
    $this->assertEquals('These are the bytes...', $image->getFile()->getBytes());
}

```

I have tracked the failure deeper to `mongoCursor->key()` in `Doctrine\MongoDB\Cursor::key()`.

I have tested the Mongo Grid FS on my system with the MongoFiles command line tool, and it is working fine.

Without error messages I have no idea where to look. Does anyone have suggestions?