formId is mandatory

#0 /home/silvano/httpdocs/app/code/core/Mage/Core/Model/Config.php(1357): Mage_Captcha_Model_Zend->__construct(Array)
#1 /home/silvano/httpdocs/app/Mage.php(466): Mage_Core_Model_Config->getModelInstance('captcha/zend', Array)
#2 /home/silvano/httpdocs/app/code/core/Mage/Captcha/Helper/Data.php(72): Mage::getModel('captcha/zend', Array)
#3 /home/silvano/httpdocs/app/code/core/Mage/Captcha/controllers/RefreshController.php(45): Mage_Captcha_Helper_Data->getCaptcha(NULL)
#4 /home/silvano/httpdocs/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Captcha_RefreshController->indexAction()
#5 /home/silvano/httpdocs/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(254): Mage_Core_Controller_Varien_Action->dispatch('index')
#6 /home/silvano/httpdocs/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#7 /home/silvano/httpdocs/app/code/core/Mage/Core/Model/App.php(381): Mage_Core_Controller_Varien_Front->dispatch()
#8 /home/silvano/httpdocs/app/Mage.php(686): Mage_Core_Model_App->run(Array)
#9 /home/silvano/httpdocs/index.php(130): Mage::run('lu', 'website')
#10 {main}