diff --git a/Sources/PersonalMessage.php b/Sources/PersonalMessage.php
index 78178400a7..d7193aa4c0 100644
--- a/Sources/PersonalMessage.php
+++ b/Sources/PersonalMessage.php
@@ -3810,7 +3810,7 @@ function ManageRules()
 		foreach ($_POST['acttype'] as $ind => $type)
 		{
 			// Picking a valid label?
-			if ($type == 'lab' && (!isset($_POST['labdef'][$ind]) || !isset($context['labels'][$_POST['labdef'][$ind]])))
+			if ($type == 'lab' && (!ctype_digit((string) $ind) || !isset($_POST['labdef'][$ind]) || !isset($context['labels'][$_POST['labdef'][$ind]])))
 				continue;
 
 			// Record what we're doing.