Warning: Javascript Window "0"

Using LC ES2 with the following JavaScript for opening attachments.  When I highlight the attachment and click Open I get a popup Warning: JavaScript Window - with a  big red X and 0.

var oObj = event.target;

var sAttachObjName = ListBox1.selectedIndex;

oObj.exportDataObject({ cName: sAttachObjName, nLaunch: 2 });