Why not use THIS.AddListItem(lc_DisplayValue, li_KeyValue)?

>I am trying to use the ItemIdData for a combo box. I have placed the following code in the INIT method
>
>SCAN
> this.AddItem(lc_DisplayValue)
> this.ItemIdData(This.NewItemID) = li_KeyValue && integer
>ENDSCAN
>
>When I step through teh code all of teh ItemIDData valuse are zero.
>
>What am I doing wrong?