Hartl Rails Tutorial 5.3.4 Integration_test

On Hartl's Rails Tutorial, the instructions are to generate a template test with $ rails generate integration_test site_layout

I am getting the error /Users/AlfonsoGiron/.rvm/gems/ruby-2.2.1/gems/sprockets-3.5.2/lib/sprockets/manifest.rb:73:in 'read': Is a directory @ io_fread - /Users/AlfonsoGiron/sample_app/public/assets (Errno::EISDIR)' followed by multiple lines of from /Users/AlfonsoGiron/.rvm/gems/ruby-2.2.1 .. etc etc

So I can see its pointing to 'assets' in the directory. Not sure what I am supposed to do though to fix the error. Any ideas?