diff --git a/CHANGES.md b/CHANGES.md
index 7acf03608649..4d0b19db3c05 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -30,6 +30,7 @@ Change Log
 * Fixed a bug where camera zooming worked incorrectly when the display height was greater than the display width [#5421](https://github.com/AnalyticalGraphicsInc/cesium/pull/5421)
 * Updated glTF/glb MIME types. [#5420](https://github.com/AnalyticalGraphicsInc/cesium/issues/5420)
 * Added `Cesium.Math.randomBetween`.
+* Modified `defaultValue` to check for both `undefined` and `null`. [#5551](https://github.com/AnalyticalGraphicsInc/cesium/pull/5551)
 
 ### 1.34 - 2017-06-01
 
diff --git a/Source/Core/defaultValue.js b/Source/Core/defaultValue.js
index 1389059d0970..011640f1d06c 100644
--- a/Source/Core/defaultValue.js
+++ b/Source/Core/defaultValue.js
@@ -19,7 +19,7 @@ define([
      * param = Cesium.defaultValue(param, 'default');
      */
     function defaultValue(a, b) {
-        if (a !== undefined) {
+        if (a !== undefined && a !== null) {
             return a;
         }
         return b;
diff --git a/Specs/Core/defaultValueSpec.js b/Specs/Core/defaultValueSpec.js
new file mode 100644
index 000000000000..19f6b98096d3
--- /dev/null
+++ b/Specs/Core/defaultValueSpec.js
@@ -0,0 +1,20 @@
+/*global defineSuite*/
+defineSuite([
+        'Core/defaultValue'
+    ], function(
+        defaultValue) {
+    'use strict';
+
+    it('Works with first parameter undefined', function() {
+        expect(defaultValue(undefined, 5)).toEqual(5);
+    });
+
+    it('Works with first parameter null', function() {
+        expect(defaultValue(null, 5)).toEqual(5);
+    });
+
+    it('Works with first parameter not undefined and not null', function() {
+        expect(defaultValue(1, 5)).toEqual(1);
+    });
+
+});