protected function getValidators($key)
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root($key);

        $rootNode
            ->defaultValue(array())
            ->prototype('variable')
            ->end()
            ->beforeNormalization()
                ->always()
                ->then(function ($values) {
                    // Normalize null as array
                    foreach ($values as $key => $value) {
                        if ($value === null) {
                            $values[$key] = array();
                        }
                    }
                    return $values;
                })
            ->end();

        $this->addValidatorValidation($rootNode);

        return $rootNode;
    }