[flexcoders] Why this does not work?

[markgoldin_2000]

I have a class based on DataGrid:
package CustomComponents
{
        import mx.collections.*;
        import mx.controls.DataGrid;
        import mx.controls.dataGridClasses.*;
        import mx.events.DataGridEvent;
        import mx.utils.ObjectUtil;
        public class sortableDataGrid extends DataGrid
        {
                public function sortableDataGrid()
                {
                        super();
                        addEventListener("headerRelease", 
headerReleaseHeandler);
                }
....
This class has a public function:
public function numericSort(itemA:Object, itemB:Object):int 
{
        return ObjectUtil.numericCompare(itemA[col], itemB[col]);
}

I am using this class in mxml class:
<customComponents:sortableDataGrid height="100%" enabled="true" 
id="TrackTotal" verticalScrollPolicy="on" 
                rowHeight="22" width="100%">
                <customComponents:columns>
                        <mx:DataGridColumn headerText="Inbound Train" 
dataField="tcotid"/>
                        <mx:DataGridColumn headerText="Block" 
dataField="tcbkcd"/>
                        <mx:DataGridColumn headerText="Position" 
dataField="tccntl" sortCompareFunction="TrackTotal.numericSort"/>
                        <mx:DataGridColumn headerText="Car Initial" 
dataField="tccari"/>
                        <mx:DataGridColumn headerText="Car Number" 
dataField="tccarn"/>
                        <mx:DataGridColumn headerText="L/E Status" 
dataField="tclest"/>
                        <mx:DataGridColumn headerText="Car Type" 
dataField="tcctyp"/>
                </customComponents:columns>
        </customComponents:sortableDataGrid>    

I am having problem with sortCompareFunction="TrackTotal.numericSort".
The compiler will not take sortCompareFunction="numericSort" and I 
dont know why.
If I use TrackTotal.numericSort then it gets compiled but the run-
time fails with:
Error: Error #1009: Cannot access a property or method of a null 
object reference.

Can someone help, please?