title: validation form return me error, $form->isValid()
body: <p>I don't understand why my form return to me a error</p>
  
  <p>This value is not valid</p>
  
  <pre><code>if ($form-&gt;isValid()) {
  var_dump($form-&gt;isValid());
  $this-&gt;get('session')
  -&gt;setFlash('notice', 'Le modifiche sono state salvate!');
  
              }else{
                  var_dump($form-&gt;getErrors());
  
              }
  </code></pre>
  
  <p>I have tried to<code>var_dump($form-&gt;getErrors());</code> return to me</p>
  
  <pre><code>array(1) { [0]=&gt; object(Symfony\Component\Form\FormError)#511 (2) { ["messageTemplate":protected]=&gt; string(23) "This value is not valid" ["messageParameters":protected]=&gt; array(0) { } } } 
  </code></pre>
  
  <p>What's wrong?</p>