diff --git a/src/Monolog/Handler/MailHandler.php b/src/Monolog/Handler/MailHandler.php
index 7d702e7d..7a7b787d 100644
--- a/src/Monolog/Handler/MailHandler.php
+++ b/src/Monolog/Handler/MailHandler.php
@@ -70,7 +70,7 @@ abstract class MailHandler extends AbstractProcessingHandler
 
     protected function isHtmlBody(string $body): bool
     {
-        return $body[0] === '<';
+        return ($body[0] ?? null) === '<';
     }
 
     /**