jQuery Grid Plugin - jqGrid

I have the following definition for my grid (great tool btw!).  The editGridRow function worked fine when I had a datatype of 'local' and added the data to the grid manually.  I changed the datatype to a function and while the grid loads correctly and the onSelectRow function is called, editGridRow just shows 'value is null' in firebug.  I appear to be missing something, but I can't figure out what.  Any ideas?

Thank you!

- Alan

$("#surveysite_list").jqGrid({

    datatype: function(postdata){
                  $.ajax ({
                        url: '<% $c->uri_for('list_data') %>',
                        data:postdata,
                        dataType: 'json',
                        complete: function(response, status) {
                            if (status == 'success'){
                                var grid = $("#surveysite_list")[0];
                                var o = eval ( '(' + response.responseText + ')' );
                                grid.addJSONData(o.data);
                            }
                        }
                  })
              },
    height: 450,
    colNames:[ 'Site Code', 'Name', 'County', 'Location', 'Position', 'Elevation', 'Comments', 'Active', 'site_id'],
    colModel:[ {name:'site_code',index:'site_code', width:100, editable: true, editrules:{required:true}, editoptions:{ size:15}, align:'center', sorttype:'integer' },
               {name:'site_name',index:'site_name', width:190, editable: true, editrules:{required:true}, editoptions:{ size:20} },
               {name:'county',index:'county', width:150, editable: true, editoptions:{ size:20} },
               {name:'location',index:'location', width:190, editable: true, editoptions:{ size:20} },
               {name:'position',index:'position', width:200, editable: true, editrules:{required:true}, editoptions:{ size:20} },
               {name:'elevation',index:'elevation', width:120, editable:true, hidden:true, editrules:{edithidden: true}, editoptions:{ size:20}},
               {name:'comments',index:'comments', width:40, editable: true, hidden: true, edittype:'textarea', editrules:{edithidden: true}, editoptions:{rows:5, cols:30} },
               {name:'active',index:'active', width:60, editable: true, edittype:'checkbox', editoptions:{value:"Yes:No", size:2}},
               {name:'site_id',index:'site_id', hidden:true, editable:true }
                ],
     sortname: 'site_code',
     sortorder: 'asc',
     multiselect: false,
     autowidth: true,
     onSelectRow: function(id) {
                      var row = $("#surveysite_list").getGridParam('selrow');
                      if ( row != null ) {
                         $("#surveysite_list").editGridRow( row, {editCaption:'Edit Survey Site Information',
                                                                navkeys: [true, 38, 40],
                                                                reloadAfterSubmit: true,
                                                                closeAfterEdit: false,
                                                                closeOnEscape: true,
                                                                afterSubmit: function(response, postdata) {
                                                                                 if (response.responseText.length > 2){
                                                                                     var problems = eval( '(' + response.responseText + ')' );
                                                                                     var message = '';
                                                                                     for ( var field in problems.errors) {
                                                                                         message += field + ": " + problems.errors[field] + "<br/>";
                                                                                         }
                                                                                     return [false, message, ""];
                                                                                 } else {
                                                                                     return [true, ""];
                                                                                 }
                                                                             }
                                                        });
                      }
                   },
     editurl: '<% $c->uri_for("update") %>',
     pager: "#pager",
     caption: "Survey Sites" }).navGrid("#pager",{edit:false, add:true, del:false, search:false});