My Calendar Plugin

Hi
I develop an issue and hopping the some one can advise me
I am running Mu Calendar on website
and I am not sure why I am getting this error

`Fatal error: Call to undefined function: memory_get_usage() in /home/content/t/r/i/tristatechess/html/tristatechess_wp/wp-content/plugins/my-calendar/my-calendar-events.php on line 345

It was working fine when I install, now I try to reinastall plugin nothing helps
Please advise

Thanx

Lev