[flexcoders] Datagrid - Sort Error

[sleblang]

I am getting the following error when attempting to run a sort on a
datagrid's dataprovider (which is an XMLListCollection). It doesn't
occur the first time I run the sort, however all subsequent runs result
in an error:
Error: Find criteria must contain at least one sort field value
Here is the sort code:
// sort on date
var sortA:Sort = new Sort();
var sortByDate:SortField = new SortField("@date",false,false)

  sortA.fields=[sortByDate];

// Refresh the collection view to show the sort.
modelLocator.projectData.projectData_xlc.sort = sortA;
modelLocator.projectData.projectData_xlc.refresh();

Any ideas as to where the error might be?

Thanks for any and all replies.