$builder->add('_captcha_token', HiddenType::class, ['data' => $captcha['token'], 'empty_data' => $captcha['token'], 'mapped' => false]);
 
                        //Add captcha
-                       $builder->add('captcha', IntegerType::class, ['label_attr' => ['class' => 'captcha'], 'label' => '<img src="'.htmlentities($captcha['src']).'" alt="'.htmlentities($captcha['equation']).'" />', 'label_html' => true, 'mapped' => false, 'translation_domain' => false]);
+                       $builder->add('captcha', IntegerType::class, ['label_attr' => ['class' => 'captcha'], 'label' => '<img src="'.htmlentities($captcha['src']).'" alt="'.htmlentities($captcha['equation']).'" />', 'label_html' => true, 'mapped' => false, 'translation_domain' => false, 'required' => true]);
 
                        //Add event listener on captcha
                        $builder->addEventListener(FormEvents::PRE_SUBMIT, [$this, 'validateCaptcha']);
                //Without captcha
                if (empty($data['captcha'])) {
                        //Add error on captcha
+                       //XXX: we need to add error on form
+                       //XXX: see https://github.com/symfony/symfony/issues/35831
                        $form->addError(new FormError($this->translator->trans('Captcha is empty')));
 
                        //Reset captcha token
                //With invalid captcha
                } elseif ($this->slugger->hash($data['captcha']) !== $data['_captcha_token']) {
                        //Add error on captcha
+                       //XXX: we need to add error on form
+                       //XXX: see https://github.com/symfony/symfony/issues/35831
                        $form->addError(new FormError($this->translator->trans('Captcha is invalid')));
 
                        //Reset captcha token