load static value column to shipment grid via observer

i have point index to main_table column value it works fine but when change this to expr static field it doesn’t work class company_name_Model_Shipment_Observer { public function coreBlockAbstractToHtmlBefore($observer){ $block = $observer->getEvent()->getBlock(); if($block->getId() == ‘sales_shipment_grid’){ $status = array(‘paid’ => ‘Paid’, ‘unpaid’ => ‘Un Paid’, ‘partially_paid’ => ‘Partially Paid’); $block->addColumnAfter( ‘is_paid’, array( ‘header’ => Mage::helper(‘sales’)->__(‘Payment Method’), ‘align’…