diff --git a/src/ol/structs/checksum.js b/src/ol/structs/checksum.js
index 423da745cf..ff72308ad2 100644
--- a/src/ol/structs/checksum.js
+++ b/src/ol/structs/checksum.js
@@ -1,12 +1,6 @@
 goog.provide('ol.structs.IHasChecksum');
 
 
-/**
- * @typedef {string}
- */
-ol.structs.Checksum;
-
-
 
 /**
  * @interface
diff --git a/src/ol/style/circlestyle.js b/src/ol/style/circlestyle.js
index 6f976cc39e..1c1742811d 100644
--- a/src/ol/style/circlestyle.js
+++ b/src/ol/style/circlestyle.js
@@ -29,7 +29,7 @@ ol.style.Circle = function(opt_options) {
 
   /**
    * @private
-   * @type {Array.<ol.structs.Checksum>|null}
+   * @type {Array.<string>}
    */
   this.checksums_ = null;
 
diff --git a/src/ol/style/fillstyle.js b/src/ol/style/fillstyle.js
index a54d8d2498..4fd62d77e7 100644
--- a/src/ol/style/fillstyle.js
+++ b/src/ol/style/fillstyle.js
@@ -26,9 +26,9 @@ ol.style.Fill = function(opt_options) {
 
   /**
    * @private
-   * @type {?ol.structs.Checksum}
+   * @type {string|undefined}
    */
-  this.checksum_ = null;
+  this.checksum_ = undefined;
 };
 
 
@@ -49,7 +49,7 @@ ol.style.Fill.prototype.getColor = function() {
  */
 ol.style.Fill.prototype.setColor = function(color) {
   this.color_ = color;
-  this.checksum_ = null;
+  this.checksum_ = undefined;
 };
 
 
@@ -57,7 +57,7 @@ ol.style.Fill.prototype.setColor = function(color) {
  * @inheritDoc
  */
 ol.style.Fill.prototype.getChecksum = function() {
-  if (goog.isNull(this.checksum_)) {
+  if (!goog.isDef(this.checksum_)) {
     this.checksum_ = 'f' + (!goog.isNull(this.color_) ?
         ol.color.asString(this.color_) : '-');
   }
diff --git a/src/ol/style/regularshapestyle.js b/src/ol/style/regularshapestyle.js
index 823ef86629..24f35ef5f4 100644
--- a/src/ol/style/regularshapestyle.js
+++ b/src/ol/style/regularshapestyle.js
@@ -28,7 +28,7 @@ ol.style.RegularShape = function(opt_options) {
 
   /**
    * @private
-   * @type {Array.<ol.structs.Checksum>|null}
+   * @type {Array.<string>}
    */
   this.checksums_ = null;
 
diff --git a/src/ol/style/strokestyle.js b/src/ol/style/strokestyle.js
index 419e771cd3..a679df2876 100644
--- a/src/ol/style/strokestyle.js
+++ b/src/ol/style/strokestyle.js
@@ -61,9 +61,9 @@ ol.style.Stroke = function(opt_options) {
 
   /**
    * @private
-   * @type {?ol.structs.Checksum}
+   * @type {string|undefined}
    */
-  this.checksum_ = null;
+  this.checksum_ = undefined;
 };
 
 
@@ -129,7 +129,7 @@ ol.style.Stroke.prototype.getWidth = function() {
  */
 ol.style.Stroke.prototype.setColor = function(color) {
   this.color_ = color;
-  this.checksum_ = null;
+  this.checksum_ = undefined;
 };
 
 
@@ -141,7 +141,7 @@ ol.style.Stroke.prototype.setColor = function(color) {
  */
 ol.style.Stroke.prototype.setLineCap = function(lineCap) {
   this.lineCap_ = lineCap;
-  this.checksum_ = null;
+  this.checksum_ = undefined;
 };
 
 
@@ -153,7 +153,7 @@ ol.style.Stroke.prototype.setLineCap = function(lineCap) {
  */
 ol.style.Stroke.prototype.setLineDash = function(lineDash) {
   this.lineDash_ = lineDash;
-  this.checksum_ = null;
+  this.checksum_ = undefined;
 };
 
 
@@ -165,7 +165,7 @@ ol.style.Stroke.prototype.setLineDash = function(lineDash) {
  */
 ol.style.Stroke.prototype.setLineJoin = function(lineJoin) {
   this.lineJoin_ = lineJoin;
-  this.checksum_ = null;
+  this.checksum_ = undefined;
 };
 
 
@@ -177,7 +177,7 @@ ol.style.Stroke.prototype.setLineJoin = function(lineJoin) {
  */
 ol.style.Stroke.prototype.setMiterLimit = function(miterLimit) {
   this.miterLimit_ = miterLimit;
-  this.checksum_ = null;
+  this.checksum_ = undefined;
 };
 
 
@@ -189,7 +189,7 @@ ol.style.Stroke.prototype.setMiterLimit = function(miterLimit) {
  */
 ol.style.Stroke.prototype.setWidth = function(width) {
   this.width_ = width;
-  this.checksum_ = null;
+  this.checksum_ = undefined;
 };
 
 
@@ -197,7 +197,7 @@ ol.style.Stroke.prototype.setWidth = function(width) {
  * @inheritDoc
  */
 ol.style.Stroke.prototype.getChecksum = function() {
-  if (goog.isNull(this.checksum_)) {
+  if (!goog.isDef(this.checksum_)) {
     var raw = 's' +
         (!goog.isNull(this.color_) ?
             ol.color.asString(this.color_) : '-') + ',' +