Magento Shipping Method Rate返回NULL

I created a new shipping method in Magento to use with the Onepage Checkout. The method appears on the front end and is defined, however whenever trying to retrieve the rates, it returns NULL. Specifically in Mage/Sales/Model/Quote.php, $method returns my method, but $rate = null.

Module:

protected $_code = 'module_shipping';

public function collectRates(Mage_Shipping_Model_Rate_Request $request)
{
    $result = Mage::getModel('shipping/rate_result');

    $result->append($this->_getMyShippingShippingRate());

    return $result;
}

protected function _getMyShippingShippingRate()
{
    $rate = Mage::getModel('shipping/rate_result_method');

    $rate->setCarrier($this->_code);

    $rate->setCarrierTitle($this->getConfigData('title'));

    $rate->setMethod('my_ship_fn');
    $rate->setMethodTitle('A New Method');

    $rate->setPrice(0);
    $rate->setCost(0);

    return $rate;
}

public function getAllowedMethods()
{
    return array(
        'my_ship_fn' => 'A New Method',
    );
}

Config.xml

<default>
    <carriers>
        <module_shipping>
            <active>1</active>
            <model>company_retailshipping/carrier_myshippingfn</model>
            <title>A New Method</title>
            <sort_order>10</sort_order>
            <sallowspecific>0</sallowspecific>
        </module_shipping>
    </carriers>
</default>

Please elaborate a bit more on "it returns NULL. Specifically in Mage/Sales/Model/Quote.php, $method returns my method, but $rate = null." -- perhaps give some class/line number for reference. Your code looks correct, and if it's appearing on the frontend then should be OK. I might guess that something else, another module perhaps, is interfering.