title: can't convert nil into String
body: <p>I don't know Ruby but I'm having this problem with my application. Somebody can help me? </p>
  
  <blockquote>
    <p>ActionView::TemplateError (can't convert nil into String) on line #7</p>
  </blockquote>
  
  <pre><code>4:  &lt;h3&gt;&lt;span&gt;&lt;%= opinion.opinion %&gt;&lt;/span&gt;: &lt;%= opinion.keywords.sort.join(' / ').upcase %&gt; &lt;/h3&gt;
  5:  &lt;div class="original"&gt;Estado original: &lt;span class="pos"&gt;&lt;%= opinion.opinion %&gt;&lt;/span&gt;&lt;/div&gt;
  6:  &lt;%#= highlight(simple_format(highlight_ontology(opinion)), opinion.keywords, :highlighter =&gt; '&lt;em&gt;\1&lt;/em&gt;') %&gt;
  7:  &lt;%= simple_format(highlight_ontology(opinion)) %&gt;
  8:  &lt;%= button_to_remote('POSITIVO', {:url =&gt; url_for(opinion), :method =&gt; :put, :with =&gt; '"opinion[opinion]=POSITIVE"' }, :class =&gt; :positive) %&gt;
  9:  &lt;%= button_to_remote('OBJETIVO', {:url =&gt; url_for(opinion), :method =&gt; :put, :with =&gt; '"opinion[opinion]=OBJECTIVE"' }, :class =&gt; :objective) %&gt;
  10: &lt;%= button_to_remote('NEGATIVO', {:url =&gt; url_for(opinion), :method =&gt; :put, :with =&gt; '"opinion[opinion]=NEGATIVE"' }, :class =&gt; :negative) %&gt;
  </code></pre>
  
  <p>Thanks!</p>