NoMethodError in Main#index

Showing app/views/main/index.html.erb where line #4 raised:

undefined method `to_uri' for "test":String

Extracted source (around line #4):

1: <% for column in @columns %>
2:   <div class="column_<%= column.position %>">
3:     <% for category in column.parent_categories %>
4:       <h2><%= link_to category.name, category_path(category) %></h2>
5:       <%= display_categories((category.children + category.forums).sort_by(&:name), category.display_mode) %>
6:     <% end %>
7:   </div>

RAILS_ROOT: /myapp

app/models/category.rb:34:in `permalink'
app/models/category.rb:38:in `to_param'
(eval):15:in `category_path'
app/views/main/index.html.erb:4
app/views/main/index.html.erb:3
app/views/main/index.html.erb:1:in `each'
app/views/main/index.html.erb:1