Custom admin theme - Theme's key does not correspond to required format: '//'

Question

I'm trying to create an admin theme in Magento 2 but I'm getting the error: Theme's key does not correspond to required format: '//' My registration.php contains:

<?php
/**
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::THEME,
    'adminhtml/GAPS/GSadmin',
    __DIR__
);

This looks perfectly correct. What am I doing wrong? How can I dig into it?