diff --git a/src/Symfony/Component/Notifier/Message/EmailMessage.php b/src/Symfony/Component/Notifier/Message/EmailMessage.php
index 5248b47a34f3c..3ca9ddc54cded 100644
--- a/src/Symfony/Component/Notifier/Message/EmailMessage.php
+++ b/src/Symfony/Component/Notifier/Message/EmailMessage.php
@@ -97,11 +97,14 @@ public function getOptions(): ?MessageOptionsInterface
     /**
      * @return $this
      */
-    public function transport(string $transport): self
+    public function transport(?string $transport): self
     {
         if (!$this->message instanceof Email) {
             throw new LogicException('Cannot set a Transport on a RawMessage instance.');
         }
+        if (null === $transport) {
+            return $this;
+        }
 
         $this->message->getHeaders()->addTextHeader('X-Transport', $transport);
 
diff --git a/src/Symfony/Component/Notifier/Message/SmsMessage.php b/src/Symfony/Component/Notifier/Message/SmsMessage.php
index d48f38e0cf937..013f970446e4a 100644
--- a/src/Symfony/Component/Notifier/Message/SmsMessage.php
+++ b/src/Symfony/Component/Notifier/Message/SmsMessage.php
@@ -81,7 +81,7 @@ public function getSubject(): string
     /**
      * @return $this
      */
-    public function transport(string $transport): self
+    public function transport(?string $transport): self
     {
         $this->transport = $transport;