has_many through checkbox form results in nil ids

I have two models, articles and goals connected through a third mentorships association (has_many through). I'm trying to set it up so the mentorship record is created through a checkbox in a form, which displays all goals via a checkbox. Right now the form submits and a mentorship is created with the article id, but the goal id is always nil. What am I doing wrong?

mentorships/_form.html.erb

<div class="log-form">
  <%= form_for(@mentorship) do |f| %>

    <%= label_tag 'Mark This As Helpful' %><br />
    <% current_user.goals.each do |goal| %>
      <%= check_box_tag "article[goal_ids][]", goal.id %>
      <%= goal.title %><br />
    <% end %>

    <div><%= hidden_field_tag :article_id, @article.id %></div>

    <p><%= f.submit %></p>
  <% end %>
</div>

article.rb

class Article < ApplicationRecord
  belongs_to :user, required: true
  has_many :mentorships
  has_many :goals, through: :mentorships

goal.rb

class Goal < ApplicationRecord 
  has_many :mentorships
  has_many :articles, through: :mentorships

mentorship.rb

class Mentorship < ApplicationRecord
  belongs_to :article
  belongs_to :goal
end

mentorships_controller.rb

class MentorshipsController < ApplicationController
  def new
    @mentorship = Mentorship.build
  end

  def create
    @mentorship = Mentorship.new(mentorship_params)
    @mentorship.save
  end

  private

  def mentorship_params
    params.permit(:article_id, :goal_ids => [])
  end
end