diff --git a/plugins/CasAuthentication/actions/caslogin.php b/plugins/CasAuthentication/actions/caslogin.php
index 9250b43b7a..468718b4c9 100644
--- a/plugins/CasAuthentication/actions/caslogin.php
+++ b/plugins/CasAuthentication/actions/caslogin.php
@@ -41,7 +41,7 @@ class CasloginAction extends Action
                 $this->serverError(_m('Incorrect username or password.'));
             }
 
-            if ($casSettings['user_whitelist'] != null && !in_array($user->nickname, $casSettings['user_whitelist'])) {
+            if (is_array($casSettings['user_whitelist']) && !in_array($user->nickname, $casSettings['user_whitelist'])) {
                 // TRANS: Server error displayed when trying to log in with non-whitelisted user name (when whitelists are enabled.)
                 $this->serverError(_m('Incorrect username or password.'));
             }