I tried your code and it is working fine although I was having an error for a while until I figure it out, that I only posted one short introduction post, that of course no one re-blogged, so my list of potential winners was empty and that's why program crashed.
Maybe a quick check if list of potential winners is not empty before running random may be a slight improvement :-)
Ah thanks for pointing that out @veleje . I updated it and added some validation before picking a winner :)