title: TYPO3 - “The given subject was not a collection”
body: <p>has anyone ever had an error like that: </p>
  
  <pre><code>The given subject was not a collection
  </code></pre>
  
  <p>from <code>typo3/sysext/extbase/Classes/Validation/Validator/CollectionValidator.php:57</code> (9.5.13)?  </p>
  
  <p>After submitting a form to a controller action this error is thrown before everything is passed to the controller action. The passed form data includes some custom entities used within the submitting template.</p>