Rails has_many through: (many to many relationship) error uninitialized constant Group::Keywords

A Group can have many keywords and a Keyword can have many groups. I have this relationship defined as

class Keyword < ActiveRecord::Base
   has_many :groups_keywords
   has_many :groups, through: :groups_keywords
end

class GroupsKeyword < ActiveRecord::Base
  belongs_to :groups
  belongs_to :keywords
end

class Group < ActiveRecord::Base
  has_many :groups_keywords
  has_many :keywords, through: :groups_keywords
end

I can do Group.find(1).groups_keywords so the relationship is working?

But I want to get all of my Groups keywords so I do the following.

Group.find(1).keywords

But I get the error uninitialized constant Group::Keywords