diff --git a/modules/backend/behaviors/UserPreferencesModel.php b/modules/backend/behaviors/UserPreferencesModel.php
index f552a2210..b9bb6dc01 100644
--- a/modules/backend/behaviors/UserPreferencesModel.php
+++ b/modules/backend/behaviors/UserPreferencesModel.php
@@ -18,7 +18,10 @@ use Backend\Models\UserPreferences;
  */
 class UserPreferencesModel extends SettingsModel
 {
-    protected static $instances = [];
+    /**
+     * @var array Internal cache of model objects.
+     */
+    private static $instances = [];
 
     /**
      * Constructor
diff --git a/modules/system/behaviors/SettingsModel.php b/modules/system/behaviors/SettingsModel.php
index 0603eec87..9bcea8575 100644
--- a/modules/system/behaviors/SettingsModel.php
+++ b/modules/system/behaviors/SettingsModel.php
@@ -28,7 +28,7 @@ class SettingsModel extends ModelBehavior
     /**
      * @var array Internal cache of model objects.
      */
-    protected static $instances = [];
+    private static $instances = [];
 
     /**
      * {@inheritDoc}