diff --git a/src/core/game/GameMap.ts b/src/core/game/GameMap.ts
index 6081cafa3..8694afd44 100644
--- a/src/core/game/GameMap.ts
+++ b/src/core/game/GameMap.ts
@@ -118,7 +118,7 @@ export class GameMapImpl implements GameMap {
   }
 
   isValidRef(ref: TileRef): boolean {
-    return this.isValidCoord(this.x(ref), this.y(ref));
+    return ref >= 0 && ref < this.refToX.length;
   }
 
   x(ref: TileRef): number {