Proxy checker gives question mark circle on https

[jp1]

What can this be due to? I have a bunch of private proxies which get + marks on everything except gateway, and I'm quite sure are compatible with https, but the https section of the alive proxies list says they're neither + nor - but just ?.

 

[rostonix]

There's an option for HTTPS checking in proxy source's attributes.
Choose it. Save. Re-check.
Screen is russian, but you'll get the point.

http://gyazo.com/ab4e52f211b7015a750bbb8cd058a52f