diff --git a/libraries/config/FormDisplay.class.php b/libraries/config/FormDisplay.class.php
index 725a2ad47c..94465107f0 100644
--- a/libraries/config/FormDisplay.class.php
+++ b/libraries/config/FormDisplay.class.php
@@ -75,7 +75,7 @@ class FormDisplay
 
     /**
      * Dictionary with user preferences keys
-     * @var array
+     * @var array|null
      */
     private $_userprefsKeys;