NoMethodError on deleting a file

[kicktheken]

session = RubyBox::Session.new(
  :client_id => ENV['BOX_CLIENT_ID'],
  :client_secret => ENV['BOX_CLIENT_SECRET'],
  :access_token => ENV['BOX_ACCESS_TOKEN']
)
client = RubyBox::Client.new(session)
file = client.upload_file_by_folder_id(valid_local_file_path, 0)
client.file_by_id(file.id).delete

# NoMethodError: undefined method `[]=' for nil:NilClass
# .rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/ruby-box-1.15.0/lib/ruby-box/session.rb:120:in `handle_errors'