Index: tests/Zend/Http/UserAgentTest.php
===================================================================
--- tests/Zend/Http/UserAgentTest.php	(revision 23555)
+++ tests/Zend/Http/UserAgentTest.php	(revision )
@@ -565,4 +565,17 @@
         $device    = $userAgent->getDevice();
         $groups = $device->getAllGroups();
     }
+
+    /**
+     * @group ZF-10665
+     */
+    public function testDontDieOnSerialization()
+    {
+        $config    = $this->config;
+        $userAgent = new Zend_Http_UserAgent($config);
+
+        // If this code doesn't throw a fatal error the test passed.
+        $userAgent->setUserAgent('userAgentTest');
+        $userAgent->serialize();
-}
+    }
+}
Index: library/Zend/Http/UserAgent.php
===================================================================
--- library/Zend/Http/UserAgent.php	(revision 23483)
+++ library/Zend/Http/UserAgent.php	(revision )
@@ -174,8 +174,8 @@
         $spec = array(
             'browser_type' => $this->_browserType,
             'config'       => $this->_config,
-            'device_class' => get_class($this->_device),
-            'device'       => $this->_device->serialize(),
+            'device_class' => get_class($this->getDevice()),
+            'device'       => $this->getDevice()->serialize(),
             'user_agent'   => $this->getServerValue('http_user_agent'),
             'http_accept'  => $this->getServerValue('http_accept'),
         );