Minitest uninitialized constant error

I am trying to run Minitest with Spec syntax with rake test and get this error:

/path/to/gem/spec/script_spec.rb:3:in `<top (required)>': uninitialized constant MyGem (NameError)

My Rakefile:

require 'rake/testtask'

Rake::TestTask.new do |t|
  t.test_files = FileList['spec/*_spec.rb']
end

My file structure:

gem/
--lib/
----script.rb
--spec/
----script_spec.rb
--Rakefile

My script.rb:

module MyGem
  class OptionParser
    def self.option?(arg)
      arg =~ /^-{1,2}\w+$/
    end
  end
end

Using Minitest::Spec syntax in script_spec.rb:

require "minitest/autorun"

describe MyGem::OptionParser do
  describe "option?" do
    it "must be true for option name" do
      OptionParser.option?('--nocolor').assert true
    end
  end
end

How do I fix it? Maybe lib folder isn't loaded? Do I miss something related to Spec syntax?