diff --git a/js/world.mjs b/js/world.mjs
index f5e0191..5a6116a 100644
--- a/js/world.mjs
+++ b/js/world.mjs
@@ -11,10 +11,12 @@ function create_world (size) {
 const world = create_world(BOX_SIZE);
 
 export function set_tile (world, x, y, tile) {
+	if (out_of_bounds(x, y)) return;
 	world.tiles[x][y] = tile;
 }
 
 export function get_tile (world, x, y) {
+	if (out_of_bounds(x, y)) return { type: "out_of_bounds" };
 	return world.tiles[x][y];
 }
 
@@ -25,7 +27,11 @@ export function create_box (parent) {
 	};
 }
 
-
 export function get_root () {
 	return world;
 }
+
+export function out_of_bounds (x, y) {
+	return x < 0 || x >= BOX_SIZE ||
+		y < 0 || y >= BOX_SIZE
+}