How to `gem install mysql -v 2.7`

TypeError: can't dup NilClass

If you get an error something like below while bundle install

Installing mysql (2.7) with native extensions
TypeError: can't dup NilClass
An error occurred while installing mysql (2.7), and Bundler cannot continue.
Make sure that `gem install mysql -v '2.7'` succeeds before bundling.

First, check your gem's version by executing

$ gem -v

If it's newer then 1.5.x, like 1.6.x or above, that's your problem.

Newer versions of Rubygems doesn't play well with the mysql gem.

So downgrade the gem, and retry gem install mysql -v 2.7.

You can downgrade the gem by:

$ gem list rubygems-update

*** LOCAL GEMS ***
rubygems-update (1.7.2)

$ sudo gem uninstall -v 1.7.2 rubygems-update # uninstall all the local rubygem-update versions if it's newer then 1.5.x
$ sudo gem install -v 1.5.2 rubygems-update
$ sudo update_rubygems
$ gem -v
1.5.2