条形码Zend Framework 2中的错误

Question

I have a question about Barcode generation in Zend Framework 2.

All the time I get an error :

PHP Fatal error: Class 'Zend\Loader' not found in /var/www/vhosts/mydomain.com/httpdocs/vendor/ZendFramework/library/Zend/Validator/Barcode.php on line 110

The code I use is :

    // Only the text to draw is required
    $barcodeOptions = array('text' => 'ZEND-FRAMEWORK');

    // No required options
    $rendererOptions = array();

    $imageResource = Barcode::factory(
        'code39', 'image', $barcodeOptions, $rendererOptions
    )->render();

    echo '<pre>';
    var_dump($imageResource);

When I delete the render function chained to the barcode::factory I see a 'Zend\Barcode\Renderer\Image' object. When I run the render function I get the error.

I've got the code from the Zend Framework documentation. I couldn't find good ZF2 documentation about this. Has anybody had this problem before? What can I do about it?