diff --git a/libraries/joomla/string/string.php b/libraries/joomla/string/string.php
index 2d06d771097ff..aecb4c773c109 100644
--- a/libraries/joomla/string/string.php
+++ b/libraries/joomla/string/string.php
@@ -9,7 +9,7 @@
 
 defined('JPATH_PLATFORM') or die;
 
-use Joomla\String\String;
+use Joomla\String\String as JStringBase;
 
 /**
  * String handling class for utf-8 data
@@ -19,7 +19,7 @@
  * @since       11.1
  * @deprecated  4.0  Use {@link \Joomla\String\String} instead unless otherwise noted.
  */
-abstract class JString extends String
+abstract class JString extends JStringBase
 {
 	/**
 	 * Split a string in camel case format
diff --git a/tests/unit/stubs/FormInspectors.php b/tests/unit/stubs/FormInspectors.php
index 61625fe984004..9289bc54220ad 100644
--- a/tests/unit/stubs/FormInspectors.php
+++ b/tests/unit/stubs/FormInspectors.php
@@ -7,6 +7,8 @@
  * @license     GNU General Public License version 2 or later; see LICENSE
  */
 
+use Joomla\Registry\Registry;
+
 /**
  * Inspector classes for the forms library.
  */
@@ -93,7 +95,7 @@ public function findField($name, $group = null)
 	 *
 	 * @return void
 	 */
-	public function findGroup($group)
+	public function &findGroup($group)
 	{
 		return parent::findGroup($group);
 	}
@@ -106,7 +108,7 @@ public function findGroup($group)
 	 *
 	 * @return void
 	 */
-	public function findFieldsByGroup($group = null, $nested = false)
+	public function &findFieldsByGroup($group = null, $nested = false)
 	{
 		return parent::findFieldsByGroup($group, $nested);
 	}
@@ -118,7 +120,7 @@ public function findFieldsByGroup($group = null, $nested = false)
 	 *
 	 * @return void
 	 */
-	public function findFieldsByFieldset($name)
+	public function &findFieldsByFieldset($name)
 	{
 		return parent::findFieldsByFieldset($name);
 	}
@@ -203,7 +205,7 @@ public function loadRuleType($type, $new = true)
 	 *
 	 * @return boolean
 	 */
-	public function validateField($element, $group = null, $value = null, $input = null)
+	public function validateField(SimpleXMLElement $element, $group = null, $value = null, Registry $input = null)
 	{
 		return parent::validateField($element, $group, $value, $input);
 	}
diff --git a/tests/unit/suites/libraries/joomla/cache/JCacheStorageTest.php b/tests/unit/suites/libraries/joomla/cache/JCacheStorageTest.php
index 4d846c4bdd4bb..15a8f9639ea2b 100644
--- a/tests/unit/suites/libraries/joomla/cache/JCacheStorageTest.php
+++ b/tests/unit/suites/libraries/joomla/cache/JCacheStorageTest.php
@@ -63,7 +63,7 @@ class JCacheStorageTest extends TestCase
 	 *
 	 * @return    boolean   To not continue with JError processing
 	 */
-	public static function errorCallback(&$error)
+	public static function errorCallback($error)
 	{
 		self::$actualError['code'] = $error->get('code');
 		self::$actualError['msg'] = $error->get('message');