Javascipt ใช้ document.getElementById().value ไม่ได้ครับ

<form action="" method="get" name="frmOrder" id="frmOrder">
   
<input name="ems_id" size="30" type="text" placeholder="* สถานะ Shipped ต้องใส่รหัส EMS" value="<?php echo  $od_shipping_id; ?>" />

             <select name="cboOrderStatus" id="cboOrderStatus" class="box">
                    <?php echo $orderOption; ?> </select> 
                    
 <input name="btnModify" type="button" id="btnModify" value="Modify Status" class="box" onClick="modifyOrderStatus(<?php echo $orderId; ?>);">
          
</form>

function modifyOrderStatus(orderId)
{
	statusList = window.document.frmOrder.cboOrderStatus;
	status     = statusList.options[statusList.selectedIndex].value;
	ems_id     = document.getElementById("ems_id");
	alert(ems_id);
	
}