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.