使用symfony配置树生成器创建多维数组

我想为我自己的bundle创建一个这样的配置:

my_filters:
    filter_type_a:
        - 'MyBundle'My'ClassA
        - 'MyBundle'My'ClassA2
    filter_type_b:
        - 'MyBundle'My'ClassB

my_filters应该是一个变长数组，my_filters.filter_type_a也应该是一个变长数组。

我试着

$treeBuilder = new TreeBuilder(); 
$rootNode = $treeBuilder->root('my_bundle');
$rootNode
     ->children()
         ->arrayNode('my_filters')
             ->prototype('array')
                 ->prototype('array')
                     ->children()
                         ->scalarNode('my_filters')->end()
                      ->end()
                 ->end()
             ->end()
         ->end()
     ->end()

，但我得到以下错误:Invalid type for path "my_bundle.my_filters.filter_type_a.0". Expected array, but got string .

下面是我设置配置的地方:

class MyBundleExtension extends Extension
{
    public function load(array $configs, ContainerBuilder $container)
    {
        $this->loadServices($container);
        $this->loadConfig($configs, $container);
    }
    private function loadConfig(array $configs, ContainerBuilder $container)
    {       
        $configuration = new Configuration();
        $config = $this->processConfiguration($configuration, $configs);
        $container->setParameter('my_bundle.my_filters', $config['my_filters']);
    }
    private function loadServices(ContainerBuilder $container)
    {
        $loader = new Loader'YamlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));
        $loader->load('services.yml');
    }
}