Relic Castle

I'm currently doing a little fan game project (no idea if I'll turn it into a game or if I'll just do it for fun) but when I walk to the edge of the map, I don't have anything blocking it off because I'm going to make it lead onto a route, It shows a water reflection. It's not any tiles that I placed, it's not on the map, it's outside the tile range. The map is 30x25 and so basically the "26th" tile row has a water reflection but it's the void? How do I fix this?

https://gyazo.com/80fa4707f943d8a35233486b8663669f