Warning when Deleting Image from Media Library

FYI: I have this plugin network activated on a WordPress Multisite install.

I get this weird white screen with this warning after I click to permanently delete an image from the media library:

Warning: unlink(public_html/wp-content/uploads/2014/09/): Is a directory in public_html/wp-content/plugins/ewww-image-optimizer/common.php on line 918

Warning: Cannot modify header information – headers already sent by (output started at public_html/wp-content/plugins/ewww-image-optimizer/common.php:918) in public_html/wp-includes/pluggable.php on line 1178

https://wordpress.org/plugins/ewww-image-optimizer/