diff --git a/calendar/freebusy.php b/calendar/freebusy.php
index 65f8f1533f..e20c3d63ca 100644
--- a/calendar/freebusy.php
+++ b/calendar/freebusy.php
@@ -50,6 +50,7 @@ function fail_exit($msg)
echo "\n\n$msg\n\n\n$msg
\n\n\n";
+ header('WWW-Authenticate: Basic realm="'.($GLOBALS['egw_info']['flags']['auth_realm'] ?: 'EGroupware').'"');
http_response_code(401);
exit;
}