I see quotes, -0.10 at 0.15 for the 18/19 put spread. You can buy instantly at 15 cents and sell at -0.10. Nobody in their right mind would sell that below 0, or even at zero. But it's just the quote. I would bid -0.05 for any given size.... A market maker needs to give quotes at everything, so that means bidding -,xx for something that has zero value.
The theoretical price of the put spread would probably be about 3 cents.