LoadError: cannot load such file -- zip

[kgish]

I have the following code:

Gemfile

gem 'docx', '~> 0.2.07'

Rake task

require 'docx'
 docx = Docx::Document.open(template_docx)
...

And I get the following error:

LoadError: cannot load such file -- zip

Am I doing something wrong?