diff --git a/app/lib/surface/CoordinateGrid.coffee b/app/lib/surface/CoordinateGrid.coffee
index 1d796ada2..8712f3fc7 100644
--- a/app/lib/surface/CoordinateGrid.coffee
+++ b/app/lib/surface/CoordinateGrid.coffee
@@ -24,8 +24,8 @@ module.exports = class CoordinateGrid extends CocoClass
   toString: -> '<CoordinateGrid>'
 
   build: (worldSize) ->
-    worldWidth = worldSize[0] ? 80
-    worldHeight = worldSize[1] ? 68
+    worldWidth = worldSize[0] or 80
+    worldHeight = worldSize[1] or 68
     @gridContainer = new createjs.Container()
     @gridShape = new createjs.Shape()
     @gridContainer.addChild @gridShape