Cannot set HTTP gem source: “source https://rubygems.org not present in cache”

My ruby version in Windows 10:

> ruby -v
ruby 2.3.1p112 (2016-04-26 revision 54768) [i386-mingw32]

When I list gem system sources I see both, HTTPS and HTTP sources:

> gem sources --list
*** CURRENT SOURCES ***

https://rubygems.org/
http://rubygems.org

Now I would like to leave only the HTTP source:

> gem sources --remove https://rubygems.org
source https://rubygems.org not present in cache

> gem sources --add http://rubygems.org
https://rubygems.org is recommended for security over http://rubygems.org

Do you want to add this insecure source? [yn]  y
source http://rubygems.org already present in the cache

Please help me to understand how to get the things done. They must be done.