Undefined Method "authentication" for #<Napster::Client:0x0000559185ef3cf8> - ruby-on-rails

Question

Not used to asking questions On stack, Apologies if the format makes it hard to respond.
Anyway, I'm trying to develop an app using Ruby on Rails with the Napster API. I am currently stuck on setting up the client object that will allow me to make meta data calls.
I am setting up the client in config/initializers as napster.rb. Here is my code
require 'napster'
client_hash = {
api_key: ENV["NAPSTER_API_KEY"],
api_secret: ENV["NAPSTER_API_SECRET"],
username: ENV["NAPSTER_USER"],
password: ENV["NAPSTER_PW"]
}
client = Napster::Client.new(client_hash)
client.access_token
client.authentication.access_token # => returns access_token
client.authentication.refresh_token
client.authentication.expires_in
Now whenever I try to run rails c in the console I get this error
config/initializers/napster.rb:14:in <main>: undefined method authentication' for #<Napster::Client:0x0000559185ef3cf8>
The ENV variables are stored in config/application.yml. I'm not sure what's going on, Here is the #<Napster::Client:0x0000559185ef3cf8>
#<Napster::Client:0x0000559185ef3cf8
#api_key=--Omitted--,
#api_secret=--Omitted--,
#username=--Omitted--, #password=--Omitted--,
#request=#<Napster::Request:0x0000559185ef3b40 #faraday=#
<Faraday::Connection:0x0000559185ef3a28 #parallel_manager=nil,
#headers={"Authorization"=>"Basic
Tm1KbVpHRXlOV0l0WVRJNFppMDBPVEkwTFdJM1l
6WXRPR1ExTTJSaE16WXpORE5tOllXWmlNek5oT0RFdE
5UaG1PUzAwWlRWakxXSXpNRFF0WVRJeU56bG1abUkzTmpJMA==", "User-
Agent"=>"Faraday v0.9.2"}, #params={}, #options=#
<Faraday::RequestOptions (empty)>, #ssl=#<Faraday::SSLOptions
verify=true>, #default_parallel_manager=nil, #builder=#
<Faraday::RackBuilder:0x0000559185ef3758 #handlers=
[Faraday::Request::UrlEncoded, Faraday::Adapter::NetHttp], #app=#
<Faraday::Request::UrlEncoded:0x0000559185efd050 #app=#
<Faraday::Adapter::NetHttp:0x0000559185efd0c8 #app=#
<Proc:0x0000559185efd1b8#/home/leo/.rbenv/versions/
2.4.4/lib/ruby/gems/2.4.0/gems/faraday-
0.9.2/lib/faraday/rack_builder.rb:152 (lambda)>>>>, #url_prefix=#
<URI::HTTPS https://api.napster.com/>, #proxy=nil>>,
#access_token=--Omitted--,
#refresh_token=--Omitted--,
#expires_in=86399>
I omitted the api and access token stuff for obvious security reasons. Any thoughtful input is appreciated, thanks.