diff --git a/src/ol/render/box.js b/src/ol/render/box.js
index 1dc131c591..14c80516d9 100644
--- a/src/ol/render/box.js
+++ b/src/ol/render/box.js
@@ -26,9 +26,9 @@ ol.render.Box = function(opt_style) {
 
   /**
    * @private
-   * @type {goog.events.Key|undefined}
+   * @type {goog.events.Key}
    */
-  this.postComposeListenKey_ = undefined;
+  this.postComposeListenKey_ = null;
 
   /**
    * @private
@@ -117,9 +117,9 @@ ol.render.Box.prototype.requestMapRenderFrame_ = function() {
  * @param {ol.Map} map Map.
  */
 ol.render.Box.prototype.setMap = function(map) {
-  if (goog.isDef(this.postComposeListenKey_)) {
+  if (!goog.isNull(this.postComposeListenKey_)) {
     goog.events.unlistenByKey(this.postComposeListenKey_);
-    this.postComposeListenKey_ = undefined;
+    this.postComposeListenKey_ = null;
     this.map_.requestRenderFrame();
     this.map_ = null;
   }