Right, you need to have a double equals when checking for equality:selectedRow = event.source.parent.parent.getComponent('Table_Trailers').selectedRow
if selectedRow == -1:
system.gui.messageBox("Must select Row to update")
else:
param1 = event.source.parent.parent.getComponent('Table_Trailers').data.getValueAt(selectedRow,"Record_ID")
system.nav.openWindow('YARD/Yard_Update_PopUp', {'SelectedRow' : param1})
system.nav.centerWindow('YARD/Yard_Update_PopUp')