require_tree argument must be a directory

I want to have a onepager with different css, javascript and images files to use as my landing page for my application.

I moved my css in the vendor/assets folder under a subfolder called onepager. I used a subfolder because I'll only use these files for the onepager and not for anything else.

e.g.

vendor/assets/stylesheets/onepager/

I created a seperate layout which references seperate css files. > onepager.html.erb

<%= stylesheet_link_tag    'onepager', media: 'all', 'data-turbolinks-track' => true %>
<%= javascript_include_tag 'onepager', 'data-turbolinks-track' => true %

Within the files onepager.js and onepager.css.scss I want them to reference to the files within the vendor/assets/xxx/onepager/ folder. I have added following code but I get an error.

onepager.css.scss

/*
* This is a manifest file that'll be compiled into application.css, which will include all the files
* listed below.
*
* Any CSS and SCSS file within this directory, lib/assets/stylesheets, vendor/assets/stylesheets,
* or vendor/assets/stylesheets of plugins, if any, can be referenced here using a relative path.
*
* You're free to add application-wide styles to this file and they'll appear at the bottom of the
* compiled file so the styles you add here take precedence over styles defined in any styles
* defined in the other CSS/SCSS files in this directory. It is generally better to create a new
* file per style scope.
*
*= require_tree ./onepager/
*/

and

onepager.js

/*
* This is a manifest file that'll be compiled into application.css, which will include all the files
* listed below.
*
* Any CSS and SCSS file within this directory, lib/assets/stylesheets, vendor/assets/stylesheets,
* or vendor/assets/stylesheets of plugins, if any, can be referenced here using a relative path.
*
* You're free to add application-wide styles to this file and they'll appear at the bottom of the
* compiled file so the styles you add here take precedence over styles defined in any styles
* defined in the other CSS/SCSS files in this directory. It is generally better to create a new
* file per style scope.
*
*= require_tree ./onepager/
*/

I get the error that the require tree is not a directory? This is strange since it is a directory and I assume the files within the vendor assets are loaded automatically?

How can I fix this?

require_tree argument must be a directory
  (in /home/ubuntu/workspace/app/assets/stylesheets/onepager.css.scss:13)