Undefined variable: class in Zend/ModuleManager/Listener/ServiceListener.php

How to reproduce error : 

Add in config
```php
array(
    'controller_plugins' => array(
        'configuration_classes' => array(
            'My\Unfound\Controller\PluginConfig'
        )
    ),
);
```

Well, it's a little useless because I fix an error, that occur on fatal error, but the exception message is at least more readable.