diff --git a/src/ol/image.js b/src/ol/image.js
index 6b748b46c3..39bfcfe5ac 100644
--- a/src/ol/image.js
+++ b/src/ol/image.js
@@ -38,7 +38,7 @@ ol.Image = function(extent, resolution, pixelRatio, attributions, src,
    * @type {Image}
    */
   this.image_ = new Image();
-  if (!goog.isNull(crossOrigin)) {
+  if (crossOrigin) {
     this.image_.crossOrigin = crossOrigin;
   }
 
@@ -129,7 +129,7 @@ ol.Image.prototype.load = function() {
   if (this.state == ol.ImageState.IDLE) {
     this.state = ol.ImageState.LOADING;
     this.changed();
-    goog.asserts.assert(goog.isNull(this.imageListenerKeys_),
+    goog.asserts.assert(!this.imageListenerKeys_,
         'this.imageListenerKeys_ should be null');
     this.imageListenerKeys_ = [
       goog.events.listenOnce(this.image_, goog.events.EventType.ERROR,
@@ -148,7 +148,7 @@ ol.Image.prototype.load = function() {
  * @private
  */
 ol.Image.prototype.unlistenImage_ = function() {
-  goog.asserts.assert(!goog.isNull(this.imageListenerKeys_),
+  goog.asserts.assert(this.imageListenerKeys_,
       'this.imageListenerKeys_ should not be null');
   this.imageListenerKeys_.forEach(goog.events.unlistenByKey);
   this.imageListenerKeys_ = null;
diff --git a/src/ol/imagecanvas.js b/src/ol/imagecanvas.js
index 0d1920d304..743af73486 100644
--- a/src/ol/imagecanvas.js
+++ b/src/ol/imagecanvas.js
@@ -78,7 +78,7 @@ ol.ImageCanvas.prototype.handleLoad_ = function(err) {
  */
 ol.ImageCanvas.prototype.load = function() {
   if (this.state == ol.ImageState.IDLE) {
-    goog.asserts.assert(!goog.isNull(this.loader_));
+    goog.asserts.assert(this.loader_, 'this.loader_ must be set');
     this.state = ol.ImageState.LOADING;
     this.changed();
     this.loader_(goog.bind(this.handleLoad_, this));
diff --git a/src/ol/imagetile.js b/src/ol/imagetile.js
index d30cbd5450..f1e267f2b0 100644
--- a/src/ol/imagetile.js
+++ b/src/ol/imagetile.js
@@ -37,7 +37,7 @@ ol.ImageTile = function(tileCoord, state, src, crossOrigin, tileLoadFunction) {
    * @type {Image}
    */
   this.image_ = new Image();
-  if (!goog.isNull(crossOrigin)) {
+  if (crossOrigin) {
     this.image_.crossOrigin = crossOrigin;
   }
 
@@ -141,7 +141,7 @@ ol.ImageTile.prototype.load = function() {
   if (this.state == ol.TileState.IDLE) {
     this.state = ol.TileState.LOADING;
     this.changed();
-    goog.asserts.assert(goog.isNull(this.imageListenerKeys_),
+    goog.asserts.assert(!this.imageListenerKeys_,
         'this.imageListenerKeys_ should be null');
     this.imageListenerKeys_ = [
       goog.events.listenOnce(this.image_, goog.events.EventType.ERROR,
@@ -160,7 +160,7 @@ ol.ImageTile.prototype.load = function() {
  * @private
  */
 ol.ImageTile.prototype.unlistenImage_ = function() {
-  goog.asserts.assert(!goog.isNull(this.imageListenerKeys_),
+  goog.asserts.assert(this.imageListenerKeys_,
       'this.imageListenerKeys_ should not be null');
   this.imageListenerKeys_.forEach(goog.events.unlistenByKey);
   this.imageListenerKeys_ = null;