soap product quantity not updating return exceptions

Question

In magento1.9 - I am trying to update product quantity using soap but it return error here is my code

 public function update($data){
    //return $data->sku;
    $_sku = $data->sku;
    $_catalog = Mage::getModel('catalog/product');
    $_productId = $_catalog->getIdBySku($_sku);
    $product = Mage::getModel('catalog/product')->load($_productId);
    /*$product->setStoreId($this->_getStoreId())
        ->load($productId);*/
    //return $product->getData();



    if (!$product->getId()) {
        $this->_fault('not_exists');
    }

    if (!$stockData = $product->getStockData()) {
        $stockData = array();
    }

    if (isset( $data->qty)) {
        $stockData['qty'] =  $data->qty;
    }

    if (isset($data->is_in_stock)) {
        $stockData['is_in_stock'] = $data->is_in_stock;
    }


    $product->setStockData($stockData);

    try {
        $product->save();
    } catch (Mage_Core_Exception $e) {
        $this->_fault('not_updated', $e->getMessage());
    }

    return $stockData;

}

then i am getting below erros :

==> Error: Uncaught SoapFault exception: [1] Internal Error. Please see log for details. in C:\xampp\htdocs\shubhajay_magentonew\app\code\core\Mage\Api\Model\Server\Adapter\Soap.php:196 Stack trace: #0 C:\xampp\htdocs\shubhajay_magentonew\app\code\core\Mage\Api\Model\Server\Handler\Abstract.php(139): Mage_Api_Model_Server_Adapter_Soap->fault('1', 'Internal Error....') #1 C:\xampp\htdocs\shubhajay_magentonew\app\code\core\Mage\Api\Model\Server\Handler\Abstract.php(48): Mage_Api_Model_Server_Handler_Abstract->_fault('internal') #2 [internal function]: Mage_Api_Model_Server_Handler_Abstract->handlePhpError(256, 'Some transactio...', 'C:\xampp\htdocs...', 4039, Array) #3 C:\xampp\htdocs\shubhajay_magentonew\lib\Varien\Db\Adapter\Pdo\Mysql.php(4039): trigger_error('Some transactio...', 256) #4 [internal function]: Varien_Db_Adapter_Pdo_Mysql->__destruct() #5 {main} thrown