title: isValid in zend framework form returns FALSE
body: <p>I have zend framework controller.
  In init method i create a form and fill the drop-down box with</p>
  
  <pre><code>$form = new FORM_NAME();
  $form-&gt;getElement('ZdGroup')-&gt;addMultiOptions($zendesk_groups);
  </code></pre>
  
  <p>then in action i check</p>
  
  <pre><code>$formData = $this-&gt;getRequest()-&gt;getParams();
      if ($form-&gt;isValid($formData)) {
          ...
  }
  </code></pre>
  
  <p>but <code>isValid()</code> returns <code>FALSE</code>
  if I delete this line 
  <code>$form-&gt;getElement('ZdGroup')-&gt;addMultiOptions($zendesk_groups);</code>
  it return <code>TRUE</code>.</p>
  
  <p>I don't understand why, does anybody have an idea?</p>