I have fixed the issue - it turns out I only had one cell in the row object. It's meant to be an array of cells, even if there's only one cell in that array.

I am trying to set up a subgrid and for now I am just trying with one column. Here's the output of my controller action:

{"row":{"cell":["Test Breeder 2"]}}

and here's the javascript code:

    $("#grid").jqGrid({
        url: "/Entry/EntryListingByBreed/",
        datatype: "json",
        mtype: "POST",
        postData: {...},
        colNames: [...],
        colModel: [
            ...some columns
        ],
        pager: jQuery("#pager"),
        rowNum: 20,
        rowList: [10, 20, 40],
        altRows: true,
        altclass: "gridAltRow",
        height: "100%",
        viewrecords: true,
        caption: "Entries for " + breedName,
        subGrid: true,
        subGridUrl: "/Entry/GetEntrySummary/",
        subGridModel: [
        {
            name: ["Breeder"],
            width: ["500"],
            align: ["left"]
        }]
    });

Currently the subgrid is only showing the column header but no data. Any help would be greatly appreciated.
Thanks,
Dany.