jQuery Grid Plugin - jqGrid

I am trying to create a new grid in response to the results from an AJAX call where the column layout of the grid is defined on the fly.

So far I have:

  var grid = $("#results_grid");

  grid.jqGrid({

    datatype: 'local',

    colNames: column_names,

    colModel: column_model,

    caption: "Query results",

    altrows: true,

    viewrecords: true

  });

And then I add the data using:

  grid.jqGrid('addRowData', idx, data);

The problem is that I get an exception in addRowData to the effect that 't.rows.length' is null ... which it is.  After that the grid appears on the page with the columns defined, but no rows visible.

I've tried this in 3.6.4 and the 3.7 beta. Am I missing a step somewhere?