diff --git a/classes/pref/labels.php b/classes/pref/labels.php
index 2cdb919ce..0eb88ea36 100644
--- a/classes/pref/labels.php
+++ b/classes/pref/labels.php
@@ -160,7 +160,7 @@ class Pref_Labels extends Handler_Protected {
 
 	function add() {
 		$caption = clean($_REQUEST["caption"]);
-		$output = clean($_REQUEST["output"]);
+		$output = clean($_REQUEST["output"] ?? false);
 
 		if ($caption) {
 			if (Labels::create($caption)) {