Ruby on Rails

I get the following error:

2015-05-17T17:25:40.349230+00:00 app[web.1]: Started GET "/movies/1" for
160.9.0.153 at 2015-05-17 17:25:40 +0000
2015-05-17T17:25:40.423697+00:00 app[web.1]: Completed 500 Internal
Server Error in 67ms
2015-05-17T17:25:40.355931+00:00 app[web.1]: Processing by
MoviesController#show as HTML
2015-05-17T17:25:40.425095+00:00 app[web.1]:
2015-05-17T17:25:40.406984+00:00 app[web.1]:   Movie Load (2.4ms)
SELECT  "movies".* FROM "movies" WHERE "movies"."id" = $1 LIMIT 1
[["id", 1]]
2015-05-17T17:25:40.425098+00:00 app[web.1]: NoMethodError (undefined
method `sismember' for nil:NilClass):
2015-05-17T17:25:40.425100+00:00 app[web.1]:   app/models/movie.rb:20:in
`cart_action'
2015-05-17T17:25:40.425102+00:00 app[web.1]:
2015-05-17T17:25:40.425101+00:00 app[web.1]:
app/controllers/movies_controller.rb:9:in `show'
2015-05-17T17:25:40.425104+00:00 app[web.1]:
2015-05-17T17:25:41.951208+00:00 heroku[router]: at=info method=GET
path="/movies/4" host=hidden-savannah-5835.herokuapp.com
request_id=cbb36b63-06ee-4da0-992d-6d512d431ea4 fwd="160.9.0.153"
dyno=web.1 connect=0ms service=17ms status=500 bytes=1714
2015-05-17T17:25:41.932183+00:00 app[web.1]: Started GET "/movies/4" for
160.9.0.153 at 2015-05-17 17:25:41 +0000
2015-05-17T17:25:41.941428+00:00 app[web.1]: Completed 500 Internal
Server Error in 5ms
2015-05-17T17:25:41.935883+00:00 app[web.1]: Processing by
MoviesController#show as HTML
2015-05-17T17:25:41.935891+00:00 app[web.1]:   Parameters: {"id"=>"4"}
2015-05-17T17:25:41.939396+00:00 app[web.1]:   Movie Load (2.2ms)
SELECT  "movies".* FROM "movies" WHERE "movies"."id" = $1 LIMIT 1
[["id", 4]]
2015-05-17T17:25:41.944520+00:00 app[web.1]:
2015-05-17T17:25:41.944523+00:00 app[web.1]: NoMethodError (undefined
method `sismember' for nil:NilClass):
2015-05-17T17:25:41.944525+00:00 app[web.1]:   app/models/movie.rb:20:in
`cart_action'
2015-05-17T17:25:41.944526+00:00 app[web.1]:
app/controllers/movies_controller.rb:9:in `show'
2015-05-17T17:25:41.944528+00:00 app[web.1]:
2015-05-17T17:25:41.944529+00:00 app[web.1]:

movie.rb (model)

class Movie < ActiveRecord::Base
  has_many :purchases
  has_many :buyers, through: :purchases

  before_save :embed_video_url

  def poster
    "http://ia.media-imdb.com/images/M/#{poster_url}"
  end

  def imdb
    "http://www.imdb.com/title/#{imdb_id}/"
  end

  def embed_video_url
    self.video_url =
"//www.youtube.com/embed/#{video_url.split('v=')[1].split('&list')[0]}"
  end

  def cart_action(current_user_id)
    if $redis.sismember "cart#{current_user_id}", id
      "Remove from"
    else
      "Add to"
    end
  end
end

movies_controller.rb

class MoviesController < ApplicationController

  def index
    @movies = Movie.all
  end

  def show
    @movie = Movie.find(params[:id])
    @cart_action = @movie.cart_action current_user.try :id
  end

end

The weird about this error is that the application run without any
problem locally but when I deployed to heroku I can see the first page
but when I click to a movie or trying to log in i get error.

Any help?