Please help – Error

Hi!
I really, really like this plugin and I consider to purchase the premium version. But in the last couple of days the plugin has been causing problem on my site.

I get a 500 error message and in the error log I could see (on several occasions) that it was your plugin that caused it.

[17-Apr-2017 08:08:24 UTC] PHP Fatal error: Call to undefined function is_user_logged_in() in /home/filmtopp/public_html/wp-content/plugins/wp-fastest-cache/inc/cache.php on line 416

What to do about it?
Please help.