diff --git a/CHANGES.md b/CHANGES.md
index 127634c13ccb..500179bfd927 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -7,6 +7,7 @@ Change Log
 * Added ability to support touch event in Imagery Layers Split demo application. [#5948](https://github.com/AnalyticalGraphicsInc/cesium/pull/5948)
 * Fixed `Invalid asm.js: Invalid member of stdlib` console error by recompiling crunch.js with latest emscripten toolchain. [#5847](https://github.com/AnalyticalGraphicsInc/cesium/issues/5847)
 * Added CZML support for `polyline.depthFailMaterial`, `label.scaleByDistance`, `distanceDisplayCondition`, and `disableDepthTestDistance`. [#5986](https://github.com/AnalyticalGraphicsInc/cesium/pull/5986)
+* Fixed handling of KMZ files with missing `xsi` namespace declarations. [#6003](https://github.com/AnalyticalGraphicsInc/cesium/pull/6003)
 * Fixed a bug where models with animations of different lengths would cause an error. [#5694](https://github.com/AnalyticalGraphicsInc/cesium/issues/5694)
 * Added a `clampAnimations` parameter to `Model` and `Entity.model`. Setting this to `false` allows different length animations to loop asynchronously over the duration of the longest animation.
 
diff --git a/Source/DataSources/KmlDataSource.js b/Source/DataSources/KmlDataSource.js
index 9f8388e3a223..ffe1fb1ef593 100644
--- a/Source/DataSources/KmlDataSource.js
+++ b/Source/DataSources/KmlDataSource.js
@@ -284,6 +284,7 @@ define([
 
     function loadXmlFromZip(reader, entry, uriResolver, deferred) {
         entry.getData(new zip.TextWriter(), function(text) {
+            text = insertNamespaces(text);
             uriResolver.kml = parser.parseFromString(text, 'application/xml');
             deferred.resolve();
         });
diff --git a/Specs/Data/KML/undeclaredNamespaces.kmz b/Specs/Data/KML/undeclaredNamespaces.kmz
new file mode 100644
index 000000000000..0c2397950d11
Binary files /dev/null and b/Specs/Data/KML/undeclaredNamespaces.kmz differ
diff --git a/Specs/DataSources/KmlDataSourceSpec.js b/Specs/DataSources/KmlDataSourceSpec.js
index d5daecf6fdb3..2bdfb3d2279a 100644
--- a/Specs/DataSources/KmlDataSourceSpec.js
+++ b/Specs/DataSources/KmlDataSourceSpec.js
@@ -244,7 +244,7 @@ defineSuite([
         });
     });
 
-    it('load inserts missing namespace declaration', function() {
+    it('load inserts missing namespace declaration into kml', function() {
         var dataSource = new KmlDataSource(options);
         return dataSource.load('Data/KML/undeclaredNamespaces.kml').then(function(source) {
             expect(source).toBe(dataSource);
@@ -252,6 +252,14 @@ defineSuite([
         });
     });
 
+    it('load inserts missing namespace declaration into kmz', function() {
+        var dataSource = new KmlDataSource(options);
+        return dataSource.load('Data/KML/undeclaredNamespaces.kmz').then(function(source) {
+            expect(source).toBe(dataSource);
+            expect(source.entities.values.length).toEqual(1);
+        });
+    });
+
     it('load rejects nonexistent URL', function() {
         return KmlDataSource.load('test.invalid', options).otherwise(function(e) {
             expect(e).toBeInstanceOf(RequestErrorEvent);