private function registerAnnotation($class)
    {
        /** @var CustomField $annotation */
        if ($annotation = $this->annotationReader->getClassAnnotation(
            new \ReflectionClass($class),
            'Lullabot\Mpx\DataService\Annotation\CustomField'
        )) {
            if (!is_subclass_of($class, CustomFieldInterface::class)) {
                throw new \RuntimeException(sprintf('%s must implement %s.', $class, CustomFieldInterface::class));
            }

            $this->customFields[$annotation->service][$annotation->objectType][$annotation->namespace] = new DiscoveredCustomField(
                $class, $annotation
            );
        }
    }