This is going to keep haunting me... tell me - what is it that makes you think the above notice is an error?
It's a notice. It's letting you know the index doesn't exist, and it isn't affecting the function in any way (it'll just return a null value for that index). These notices should be turned off on production sites. That is the best answer; I'll fix it so this doesn't show, but that doesn't close the security hole opened by showing these notices.
Yeah, it means I've got sloppy code, I know; but it works :)
If you update the plugin, this should disappear - but do turn off display of these notices!