Imagine (imagick) can't open pdf with page parameter

Question

I use Imagine in a Silex environment,

In a main objective to create thumbnails from PDF files, I choose the romainneutron/Imagine-Silex-Service-Provider to do this kind of PHP ImageMagick conversion.

Everything is OK when I use the Imagick driver and when I open a pdf file without parameter :

bootstrap.php :

$app->register(new ImagineServiceProvider(), array('imagine.driver' => 'Imagick'));  

aController.php :

 $path = "/path/to/file/";
 $filename = "aFileame";


 $image = $app['imagine']->open(dirname(dirname(__DIR__)).$path.$filename.".pdf")
                         ->resize(new Box(160, 240))
                         ->save(dirname(dirname(__DIR__)).$path."/thumb/".$fileName."[email protected]")
                         ->resize(new Box(80, 120))
                        ->save(dirname(dirname(__DIR__)).$path."/thumb/".$filename."_thumb.png");

But if I I try to just open the first page of the document ( .pdf[0] ) :

$image = $app['imagine']->open(dirname(dirname(__DIR__)).$path.$filename.".pdf[0]")

I have got an exception :

InvalidArgumentException in AbstractImagine.php line 67:
File /path/to/file/aFileame[0] does not exist. 

Thanks for the help.
Julien