jQuery Grid Plugin - jqGrid

Hi,

Followed are two identical lines of codes, one yiels "undefined" and the other yields "36". Could you help tell me why? Thanks.

('#transGrid').jqGrid
        (   
            'navGrid',
            '#transGridPager',
            { edit:true, add:true, del:true, search:false, refresh:true},
            {
                url: 'index.php?&oId=' + $('#transGrid').jqGrid('getRowData', $('#transGrid').jqGrid('getGridParam', 'selrow'))['OrigTransId'],   <=== oId = undefined (in Firebug)
                afterShowForm: function(id)
                {   

                    //Below is the same line of code, but it outputs 36 in Firebug, which is expected
                    console.log($('#transGrid').jqGrid('getRowData', $('#transGrid').jqGrid('getGridParam', 'selrow'))['OrigTransId']);
                 }

            }

        )