Rails Controller Rspec Test

To preface, I'm extremely new to Rails and general MVC format.

I'm using Rspec to do all my testing and I'm having an issue with this piece of code in my controller_spec:

describe "GET show" do
    context 'with an admin user signed in' do
      with :admin
      let!(:invitation) { create :invitation }   
      before do
        sign_in admin
        get :show, id: invitation.id
      end
      it { should respond_with :ok }
      it { should respond_with_content_type :html }
      it { should render_template :show }
      it "assigns the requested invitation as @invitation" do
        expect(assigns(:invitations)).to eq([invitation])
      end
    end
  end

Here is the error I'm getting:

14) InvitationsController GET show with an admin user signed in assigns the requested invitation as @invitation
     Failure/Error: expect(assigns(:invitations)).to eq([invitation])

       expected: [#<Invitation id: 98, token: "blah", name: "Team", number_of_uses: 5, created_at: "2016-01-29 20:43:27", updated_at: "2016-01-29 20:43:27">]
            got: nil

       (compared using ==)
     # ./spec/controllers/invitations_controller_spec.rb:53:in `block (4 levels) in <top (required)>'

Finally, here are my controller and policy class snippets, respectively.

Controller:

class InvitationsController < ApplicationController
  before_action :set_invitation, only: [:show, :edit, :update, :destroy]

  respond_to :html

  def index
    authorize Invitation
    @invitations = policy_scope(Invitation)
    respond_with(@invitations)
  end

  def show
    authorize @invitation
    respond_with(@invitation)
  end

invitation_policy.rb:

class InvitationPolicy < ApplicationPolicy
  Scope = Struct.new(:user, :scope) do
    def resolve
      Invitation.all
    end
  end

  def index?
    user.admin?
  end

  def show?
    user.admin?
  end

The relevant gems I'm using here are Pundit and FactoryGirl. I'm still learning a lot of what these things mean, so I'm well aware that the answer may be obvious.