"Gem::LoadError (ipaddr is not part of the bundle. Add it to your Gemfile.)" after upgrading to Rails 2.5

[patbl]

In the large Rails app I work on, we started getting errors of this form when we upgraded to Ruby 2.5:

Gem::LoadError (ipaddr is not part of the bundle. Add it to your Gemfile.)

We got such errors with a few different gems: csv, date, fileutils, ipaddr, and webrick. These are all default gems.

We removed pry-debundle for other reasons, but removing it turned out to fix this error as well. I tried reproducing the issue with a new Rails app, but I couldn't. I'm creating this issue so that maybe someone who's googling for the error will find it and save some time.