Rails 5, "nil is not a valid asset source"

I don't know this is compact solution or not but this code will work.

activate the fallback method in your uploader.

  def default_url
    "/assets/fallback/" + [version_name, "default.png"].compact.join('_')
  end

Hope this will help you.

Cheers (y)

The problem was that I was trying to show an image that did not exist.

Adding unless article.image.blank? solved it:

<%= image_tag article.image_url(:thumb) unless article.image.blank? %>

EDIT: In Rails 4, this would have just rendered nothing without errors, while in Rails 5 it raises an error. So it was, in fact, an upgrade issue.

Big thanks to @BookOfGreg for pointing this out.