ListGrid sort

Code:
isc.ListGrid.create({
...
fields:[{
name:"a",
title:"a"
},{
name:"b",
title:"b",
formatCellValue : function(value, record, rowNum, colNum, grid)
{
return record["c"]["b"];
}
}]
});

record:

[{"a":"testa","c":{"b":"testB"}},...]

Click on the field B, according to the field to sort B. From the results on sorting failed.
What should I do?
thank you