Output custom field URL from CustomPress

I'm trying to use the suggested solution by the OP here:

https://premium.wpmudev.org/forums/topic/custompress-non-image-file-converted-to-file-type-thumbnail

and I've added the following to my CPT template file:

<?php wp_get_attachment_url(get_post_meta(get_the_ID(), '_ct_upload_568256198786e')[0]); ?>

However I receive the following error:

Parse error: syntax error, unexpected '['

If I remove the array reference [0] the template loads but I get no output.