I am attempting to determine which item is selected in a PtList control.

I am currently using this code:
-----------------------------------
unsigned short* pnSelPos = 0;
short*                pnItem    = 0;

PtGetResource(ABW_ListBarcodes, Pt_ARG_SELECTION_INDEXES, &pnSelPos, &pnItem);
-----------------------------------

Pt_ARG_SELECTION_MODE is set to: Pt_SINGLE_MODE

It seems that *pnSelPos identifies the currently selected item.  However, when nothing is selected *pnSelPos can still 
have a value.

Can someone explain to me what *pnSelPos and *pnItem represent in this context and if there is another way to retrieve 
which item is currently selected if any?

Thank you,
Kendall