Index: tests/Zend/Gdata/AuthSubTest.php
===================================================================
--- tests/Zend/Gdata/AuthSubTest.php	(revision 24026)
+++ tests/Zend/Gdata/AuthSubTest.php	(working copy)
@@ -125,4 +125,17 @@
             $c->setAuthSubPrivateKeyFile("zendauthsubfilenotfound",  null, true);
         }
     }
+        
+    /**
+     * @group ZF-11351
+     * @expectedException Zend_Gdata_App_HttpException
+     */
+    public function testAuthSubGetHttpClientShouldThrowExceptionOnVanillaHttpClient()
+    {
+        $client = new Zend_Http_Client();
+        $client->setUri('http://example.com/AuthSub');
+        $gdclient = Zend_Gdata_AuthSub::getHttpClient('FakeToken', $client);
+        $this->fail('Expected exception Zend_Gdata_App_HttpException not raised!');
+    }
+    
 }
Index: library/Zend/Gdata/AuthSub.php
===================================================================
--- library/Zend/Gdata/AuthSub.php	(revision 24026)
+++ library/Zend/Gdata/AuthSub.php	(working copy)
@@ -229,9 +229,9 @@
         if ($client == null) {
             $client = new Zend_Gdata_HttpClient();
         }
-        if (!$client instanceof Zend_Http_Client) {
+        if (!$client instanceof Zend_Gdata_HttpClient) {
             require_once 'Zend/Gdata/App/HttpException.php';
-            throw new Zend_Gdata_App_HttpException('Client is not an instance of Zend_Http_Client.');
+            throw new Zend_Gdata_App_HttpException('Client is not an instance of Zend_Gdata_HttpClient.');
         }
         $useragent = 'Zend_Framework_Gdata/' . Zend_Version::VERSION;
         $client->setConfig(array(