Error in Humming bird dashboard

Hello there, hope you are doing well.
I have installed Hummingbird (love the name btw) onto Telsec, however when I go to the dashboard, under the MINIFICATION section I am getting this error

Notice: Undefined index: original_size in /u/t/telsec/www.telsec.net/wp-includes/class-wp-list-util.php on line 157

Any thoughts on what the issue might be?

Thankies
Tanya