i have a grouping store that I'd like to filter by a particular field in the JSON data, but am not sure how the filtering mechanism works

here's my GroupingStore:
  var store = new Ext.data.GroupingStore({
    url: "/data/list.js",
    reader: new Ext.data.JsonReader({
      root: "rows",
      totalProperty: 'results',
        {name: 'title'},
        {name: 'filename'},
        {name: 'status'},
        {name: 'type'},
        {name: 'size'}
    sortInfo: {
      field: 'title', 
      direction: "ASC"
    groupField: 'status'
I am grouping by the status field, but that field is sometimes null. How would I filter out the nulls so that those rows don't appear in the grid? Do i do it on the GroupingStore or the JsonReader?

thanks for any tips!