diff --git a/src/MessageProcessors/Mqtt311MessageProcessor.php b/src/MessageProcessors/Mqtt311MessageProcessor.php
index 9148ab3..6a792f8 100644
--- a/src/MessageProcessors/Mqtt311MessageProcessor.php
+++ b/src/MessageProcessors/Mqtt311MessageProcessor.php
@@ -66,10 +66,10 @@ protected function parseAndValidateSubscribeAcknowledgementMessage(string $data)
     /**
      * Determines if the given message is a PUBLISH message and contains the unicode null character U+0000.
      *
-     * @param Message $message
+     * @param Message|null $message
      * @return bool
      */
-    private function isPublishMessageWithNullCharacter(Message $message): bool
+    private function isPublishMessageWithNullCharacter(?Message $message): bool
     {
         return $message !== null
             && $message->getType()->equals(MessageType::PUBLISH())