ItemStack.setAmount() changing material

Why does sandstone change into air when I use ItemStack.setAmount()?

		ItemStack I = null;
		Integer Qty = ToInt(args[2]);

			I = P.getItemInHand().clone();
			I.setAmount(Qty);

		Integer MaxQty = GetAPItem(I.getTypeId()).TransactionMaxQty;
		if (MaxQty==null) { MaxQty=1; }
		if (I.getAmount() > MaxQty) {
			Qty = MaxQty;
			MsgP(P,"ItamStack check before: "+I.getType().name());
			I.setAmount(MaxQty);
			MsgP(P,"ItamStack check after: "+I.getType().name());
		}