Fatal error in comments.php

This fatal error showed up in my logs today:

PHP Fatal error: Call to undefined function get_post_meta() in ... plugins/disqus-comment-system/comments.php

After looking into the file it does not prevent access from executing the file directly. This particular request probably came from a bot.

See here for more info on how to do this: http://codex.ww.wp.xz.cn/Writing_a_Plugin#Plugin_Files

https://ww.wp.xz.cn/plugins/disqus-comment-system/