Replying to A good riddle

I was told this about a week ago, and solved it in under 3 minutes.

There is a dragon and a knight on an island. There are also 14 wells. Every well contains poisenous water, and it kills you only a few hours after you drink it. However, the wells work on a system, so that if you drink from well number 4, you have to drive from any well numbered 5 to 14 to cancel the poison. However, the 14th well is gaurded by the dragon, so the knight can NEVER get to it.

One day, the knight and the dragon decide to have a fight to the death, but instead, they decide to each bring one glass of water, and switches glasses, and drink each other's water. After, they return back to their homes.

Later on, the dragon dies but the knight lives. How is that possible since the dragon could just give the knight water from the 14th well so he would have no way of curing himself, and the dragon could just drink from the 14th well to cancel out the poison that the knight gave him?