diff --git a/src/ol/mapbrowserevent.js b/src/ol/mapbrowserevent.js
index b5b8d49481..a2d241eda9 100644
--- a/src/ol/mapbrowserevent.js
+++ b/src/ol/mapbrowserevent.js
@@ -285,9 +285,10 @@ ol.MapBrowserEventHandler.prototype.disposeInternal = function() {
       this.handleUp_, false, this);
   goog.events.unlisten(element,
       goog.events.EventType.CLICK, this.click_, false, this);
-  goog.asserts.assert(goog.isDef(this.dragListenerKeys_));
-  goog.array.forEach(this.dragListenerKeys_, goog.events.unlistenByKey);
-  this.dragListenerKeys_ = null;
+  if (!goog.isNull(this.dragListenerKeys_)) {
+    goog.array.forEach(this.dragListenerKeys_, goog.events.unlistenByKey);
+    this.dragListenerKeys_ = null;
+  }
 };