Rails 4 - Helper not returning anything

I am on Rails 4 trying to build a simple helper to reduce some of the code in my view.

Here is the view code (show.html.erb) before using a helper:

<% unless @article.long_effects.blank? %>
  <ul>
    <% @article.long_effects.split(';').each do |effect| %>
      <li><%= effect %></li>
    <% end %>
  </ul>
<% end %>

and here is the helper I built for the above code:

def list(attribute)
  unless attribute.blank?
    content_tag(:ul) do
      attribute.split(';').each do |a|
        content_tag(:li, a)
      end
    end
  end
end

which I then call from the view like so

<%= list(@article.long_effects) %>

Unfortunately, the helper is not returning anything. This is my first time writing a helper that returns HTML, so maybe I am doing something wrong?

from

def list(attribute)
  unless attribute.blank?
    content_tag(:ul) do
      attribute.split(';').each do |a|
        content_tag(:li, a)
      end
    end
  end
end

to

def list(attribute)
  unless attribute.blank?
    content_tag(:ul) do
      attribute.split(';').each do |a|
        concat content_tag(:li, a)
      end
    end
  end
end

concat method will be useful to join the collection object from looping conditions.