When attempting to update RubyGems, depending on updates your previously performed, you might run into an error

ERROR:  While executing gem ... (TypeError)
can't convert nil into String

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

• Prevents you from data breaches and liability risks
• Upgrade at your own pace
• Works with modern Rubies

Read more Show archive.org snapshot

Fix this by running

gem uninstall rubygems-update -a -x