diff --git a/src/Monolog/Handler/SlackHandler.php b/src/Monolog/Handler/SlackHandler.php
index 4308b411..bf279bbe 100644
--- a/src/Monolog/Handler/SlackHandler.php
+++ b/src/Monolog/Handler/SlackHandler.php
@@ -184,7 +184,7 @@ class SlackHandler extends SocketHandler
         return $this->slackRecord->stringify($fields);
     }
 
-    public function setFormatter(FormatterInterface $formatter)
+    public function setFormatter(FormatterInterface $formatter): HandlerInterface
     {
         parent::setFormatter($formatter);
         $this->slackRecord->setFormatter($formatter);
diff --git a/src/Monolog/Handler/SlackWebhookHandler.php b/src/Monolog/Handler/SlackWebhookHandler.php
index a2df0f1a..6ad3e45c 100644
--- a/src/Monolog/Handler/SlackWebhookHandler.php
+++ b/src/Monolog/Handler/SlackWebhookHandler.php
@@ -90,7 +90,7 @@ class SlackWebhookHandler extends AbstractProcessingHandler
         Curl\Util::execute($ch);
     }
 
-    public function setFormatter(FormatterInterface $formatter)
+    public function setFormatter(FormatterInterface $formatter): HandlerInterface
     {
         parent::setFormatter($formatter);
         $this->slackRecord->setFormatter($formatter);