diff --git a/lib/danmu/danmu/sprite.js b/lib/danmu/danmu/sprite.js
index 05794eb..3a366c6 100644
--- a/lib/danmu/danmu/sprite.js
+++ b/lib/danmu/danmu/sprite.js
@@ -41,7 +41,7 @@ module.exports = {
 			move: function() {
 				this.x += this.speed.x;
 				this.y += this.speed.y;
-				if (this.children !== null) {
+				if (!typeof this.children == "undefined") {
 					for (var i = 0; i < this.children.length; i++) {
 						this.children[i].speed = this.speed;
 						this.children[i].move();
@@ -53,7 +53,7 @@ module.exports = {
 			 * 向此精灵添加一个子精灵
 			 */
 			appendChild: function(sprite) {
-				if (this.children === null) {
+				if (typeof this.children == "undefined") {
 					this.children = [];
 				}
 				this.children.push(sprite);
@@ -63,7 +63,7 @@ module.exports = {
 			 * 渲染子精灵
 			 */
 			drawChildren: function() {
-				if (this.children !== null) {
+				if (!typeof this.children == "undefined") {
 					for (var i = 0; i < this.children.length; i++) {
 						this.children[i].draw();
 					}