title: Undefined method 'merge' for "test":String - Rails 3.1
body: <p>I have a collection_select:</p>
  
  <pre><code>&lt;%= form_for(@feedback) do |f| %&gt;
  
  &lt;div class="field"&gt;
  &lt;%= f.label :poster_id %&gt;&lt;br /&gt;
  &lt;%= f.collection_select :feedback, :poster_id, @users, :id, @user.username, :prompt =&gt; "Select your username" %&gt;&lt;br /&gt;
  &lt;%= f.number_field :poster_id %&gt;
  &lt;/div&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>This is the error message:</p>
  
  <pre><code>ActionView::Template::Error (undefined method `merge' for "test":String):
      15:     
      16:   &lt;div class="field"&gt;
      17:     &lt;%= f.label :poster_id %&gt;&lt;br /&gt;
      18:     &lt;%= f.collection_select :feedback, :poster_id, @users, :id, @user.username %&gt;&lt;br /&gt;
      19:     &lt;%= f.number_field :poster_id %&gt;
      20:   &lt;/div&gt;
      21:   &lt;div class="field"&gt;
    app/views/feedbacks/_form.html.erb:18:in `block in _app_views_feedbacks__form_html_erb__3181571289116259961_2154876620'
    app/views/feedbacks/_form.html.erb:3:in `_app_views_feedbacks__form_html_erb__3181571289116259961_2154876620'
  </code></pre>
  
  <p>"test" is the username that was returned by <code>@user.username</code>.</p>
  
  <p>How do I fix that?</p>