diff --git a/src/ol/image.js b/src/ol/image.js
index d98229bc43..8f4245b004 100644
--- a/src/ol/image.js
+++ b/src/ol/image.js
@@ -114,7 +114,7 @@ ol.Image.prototype.handleImageError_ = function() {
  * @private
  */
 ol.Image.prototype.handleImageLoad_ = function() {
-  if (!goog.isDef(this.resolution)) {
+  if (this.resolution === undefined) {
     this.resolution = ol.extent.getHeight(this.extent) / this.image_.height;
   }
   this.state = ol.ImageState.LOADED;
diff --git a/src/ol/proj/proj.js b/src/ol/proj/proj.js
index 2109dbc766..821b133e8d 100644
--- a/src/ol/proj/proj.js
+++ b/src/ol/proj/proj.js
@@ -152,7 +152,7 @@ ol.proj.Projection = function(options) {
       if (goog.isDef(def.axis) && !goog.isDef(options.axisOrientation)) {
         this.axisOrientation_ = def.axis;
       }
-      if (!goog.isDef(options.units)) {
+      if (options.units === undefined) {
         var units = def.units;
         if (units === undefined) {
           if (def.to_meter !== undefined) {
diff --git a/src/ol/source/clustersource.js b/src/ol/source/clustersource.js
index b0aaf7f15e..d1e15c01c3 100644
--- a/src/ol/source/clustersource.js
+++ b/src/ol/source/clustersource.js
@@ -103,7 +103,7 @@ ol.source.Cluster.prototype.onSourceChange_ = function() {
  * @private
  */
 ol.source.Cluster.prototype.cluster_ = function() {
-  if (!goog.isDef(this.resolution_)) {
+  if (this.resolution_ === undefined) {
     return;
   }
   this.features_.length = 0;
diff --git a/src/ol/source/imagewmssource.js b/src/ol/source/imagewmssource.js
index ec126ecbd6..06e36fcb85 100644
--- a/src/ol/source/imagewmssource.js
+++ b/src/ol/source/imagewmssource.js
@@ -142,7 +142,7 @@ ol.source.ImageWMS.prototype.getGetFeatureInfoUrl =
   goog.asserts.assert(!('VERSION' in params),
       'key VERSION is not allowed in params');
 
-  if (!goog.isDef(this.url_)) {
+  if (this.url_ === undefined) {
     return undefined;
   }
 
@@ -188,7 +188,7 @@ ol.source.ImageWMS.prototype.getParams = function() {
 ol.source.ImageWMS.prototype.getImage =
     function(extent, resolution, pixelRatio, projection) {
 
-  if (!goog.isDef(this.url_)) {
+  if (this.url_ === undefined) {
     return null;
   }
 
diff --git a/src/ol/style/fillstyle.js b/src/ol/style/fillstyle.js
index 74f5162b62..19895f0c86 100644
--- a/src/ol/style/fillstyle.js
+++ b/src/ol/style/fillstyle.js
@@ -58,7 +58,7 @@ ol.style.Fill.prototype.setColor = function(color) {
  * @inheritDoc
  */
 ol.style.Fill.prototype.getChecksum = function() {
-  if (!goog.isDef(this.checksum_)) {
+  if (this.checksum_ === undefined) {
     this.checksum_ = 'f' + (!goog.isNull(this.color_) ?
         ol.color.asString(this.color_) : '-');
   }
diff --git a/src/ol/style/strokestyle.js b/src/ol/style/strokestyle.js
index e5599ab044..1fbd7a9a02 100644
--- a/src/ol/style/strokestyle.js
+++ b/src/ol/style/strokestyle.js
@@ -203,7 +203,7 @@ ol.style.Stroke.prototype.setWidth = function(width) {
  * @inheritDoc
  */
 ol.style.Stroke.prototype.getChecksum = function() {
-  if (!goog.isDef(this.checksum_)) {
+  if (this.checksum_ === undefined) {
     var raw = 's' +
         (!goog.isNull(this.color_) ?
             ol.color.asString(this.color_) : '-') + ',' +
diff --git a/src/ol/tilegrid/tilegrid.js b/src/ol/tilegrid/tilegrid.js
index 5cf1a4b15b..7efb55ea90 100644
--- a/src/ol/tilegrid/tilegrid.js
+++ b/src/ol/tilegrid/tilegrid.js
@@ -552,7 +552,7 @@ ol.tilegrid.createXYZ = function(opt_options) {
   var options = /** @type {olx.tilegrid.TileGridOptions} */ ({});
   goog.object.extend(options, goog.isDef(opt_options) ?
       opt_options : /** @type {olx.tilegrid.XYZOptions} */ ({}));
-  if (!goog.isDef(options.extent)) {
+  if (options.extent === undefined) {
     options.extent = ol.proj.get('EPSG:3857').getExtent();
   }
   options.resolutions = ol.tilegrid.resolutionsFromExtent(
diff --git a/test/spec/ol/extent.test.js b/test/spec/ol/extent.test.js
index b1e8578ceb..1c0235a69c 100644
--- a/test/spec/ol/extent.test.js
+++ b/test/spec/ol/extent.test.js
@@ -837,7 +837,7 @@ describe('ol.extent', function() {
     it('takes arbitrary function', function() {
       var transformFn = function(input, output, opt_dimension) {
         var dimension = goog.isDef(opt_dimension) ? opt_dimension : 2;
-        if (!goog.isDef(output)) {
+        if (output === undefined) {
           output = new Array(input.length);
         }
         var n = input.length;