Payment methods "Wrong content type" after upgrade Magento from 1.6 to 1.9.1.1

Question

I've just upgraded magento version from 1.6 to 1.9.1.1. I go to system/configuration/sales/payment methods and got the following error:

I have tried to debug by going to this function on app/code/core/Mage/Usa/Model/Shipping/Carrier/Dhl/International.php:

public function getAllowedMethods()
    {
        $contentType = $this->getConfigData('content_type');
        $allowedMethods = array();
        if ($this->_isDomestic) {
            $allowedMethods = array_merge(explode(',', $this->getConfigData('doc_methods')),
                explode(',', $this->getConfigData('nondoc_methods'))
            );
        } else {
            switch ($contentType) {
                case self::DHL_CONTENT_TYPE_DOC:
                    $allowedMethods = explode(',', $this->getConfigData('doc_methods'));
                    break;
                case self::DHL_CONTENT_TYPE_NON_DOC:
                    $allowedMethods = explode(',', $this->getConfigData('nondoc_methods'));
                    break;
                default:
                    Mage::throwException(Mage::helper('usa')->__('Wrong Content Type.'));
            }
        }
        $methods = array();
        foreach ($allowedMethods as $method) {
            $methods[$method] = $this->getDhlProductTitle($method);
        }
        return $methods;
    } 

I tried to var_dump($contentType) add get NULL value. What's wrong here ?

UPDATE:

My Content Type config in admin panel:

Answer 1

As you may have noticed in the given source code, the exception is thrown because null is not an allowed value for $contentType. $contentType is read from the system configuration:

$contentType = $this->getConfigData('content_type');

It is as simple as changing the system configuration setting to one of the allowed values. This can be done for instance via Admin Panel

System → Configuration → Sales → Shipping Methods → DHL → Content Type

or by adding a valid core_config_data entry for the config path carriers/dhlint/content_type.

Answer 2

I also get the same problem and resolved with following: Goto app > code > core > Mage > Usa > etc > Config.xml Add Below code at <carriers> <dhlint> :

<content_type>D</content_type>