diff --git a/tests/phpunit/includes/testcase-ajax.php b/tests/phpunit/includes/testcase-ajax.php
index b415976f21..e48647201f 100644
--- a/tests/phpunit/includes/testcase-ajax.php
+++ b/tests/phpunit/includes/testcase-ajax.php
@@ -50,6 +50,21 @@ abstract class WP_Ajax_UnitTestCase extends WP_UnitTestCase {
 		'wp-remove-post-lock', 'dismiss-wp-pointer', 'heartbeat', 'nopriv_heartbeat',
 	);
 
+	/**
+	 * Don't preserve the global state when using an external object cache.
+	 *
+	 * Most of the Ajax tests are running in a separate PHP process, and thus PHPUnit attempts
+	 * to preserve the global state from the parent process by serializing all globals.
+	 * But this doesn't work for external object caches so we have to disable this "feature".
+	 */
+	public function run( PHPUnit_Framework_TestResult $result = NULL ) {
+		if ( wp_using_ext_object_cache() ) {
+			$this->setPreserveGlobalState( false );
+		}
+
+		return parent::run( $result );
+	}
+
 	/**
 	 * Set up the test fixture.
 	 * Override wp_die(), pretend to be ajax, and suppres E_WARNINGs
diff --git a/tests/phpunit/tests/ajax/Response.php b/tests/phpunit/tests/ajax/Response.php
index c987bf7ccb..9c48a8161d 100644
--- a/tests/phpunit/tests/ajax/Response.php
+++ b/tests/phpunit/tests/ajax/Response.php
@@ -6,6 +6,7 @@
  * @subpackage UnitTests
  * @since      3.5.0
  * @group      ajax
+ * @runInSeparateProcess
  */
 class Tests_Ajax_Response extends WP_UnitTestCase {
 
@@ -64,7 +65,6 @@ class Tests_Ajax_Response extends WP_UnitTestCase {
 	 * Needs a separate process to get around the headers/output from the
 	 * bootstrapper
 	 * @ticket 19448
-	 * @runInSeparateProcess
 	 */
 	public function test_response_charset_in_header() {