diff --git a/src/Symfony/Bundle/FrameworkBundle/Resources/config/messenger.xml b/src/Symfony/Bundle/FrameworkBundle/Resources/config/messenger.xml
index 14ffd4d3827a..85a1a432615d 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Resources/config/messenger.xml
+++ b/src/Symfony/Bundle/FrameworkBundle/Resources/config/messenger.xml
@@ -106,6 +106,7 @@
         <!-- routable message bus -->
         <service id="messenger.routable_message_bus" class="Symfony\Component\Messenger\RoutableMessageBus">
             <argument /> <!-- Message bus locator -->
+            <argument type="service" id="messenger.default_bus" />
         </service>
     </services>
 </container>
diff --git a/src/Symfony/Component/Messenger/RoutableMessageBus.php b/src/Symfony/Component/Messenger/RoutableMessageBus.php
index cb1e2ae6273a..c0f7eeca1b9a 100644
--- a/src/Symfony/Component/Messenger/RoutableMessageBus.php
+++ b/src/Symfony/Component/Messenger/RoutableMessageBus.php
@@ -28,13 +28,15 @@
 class RoutableMessageBus implements MessageBusInterface
 {
     private $busLocator;
+    private $fallbackBus;
 
     /**
      * @param ContainerInterface $busLocator A locator full of MessageBusInterface objects
      */
-    public function __construct(ContainerInterface $busLocator)
+    public function __construct(ContainerInterface $busLocator, MessageBusInterface $fallbackBus = null)
     {
         $this->busLocator = $busLocator;
+        $this->fallbackBus = $fallbackBus;
     }
 
     public function dispatch($envelope, array $stamps = []): Envelope
@@ -43,14 +45,28 @@ public function dispatch($envelope, array $stamps = []): Envelope
             throw new InvalidArgumentException('Messages passed to RoutableMessageBus::dispatch() must be inside an Envelope');
         }
 
-        /** @var BusNameStamp $busNameStamp */
+        return $this->getMessageBus($envelope)->dispatch($envelope, $stamps);
+    }
+
+    private function getMessageBus(Envelope $envelope): MessageBusInterface
+    {
+        /** @var BusNameStamp|null $busNameStamp */
         $busNameStamp = $envelope->last(BusNameStamp::class);
-        $busName = null !== $busNameStamp ? $busNameStamp->getBusName() : MessageBusInterface::class;
+
+        if (null === $busNameStamp) {
+            if (null === $this->fallbackBus) {
+                throw new InvalidArgumentException(sprintf('Envelope is missing a BusNameStamp and no fallback message bus is configured on RoutableMessageBus.'));
+            }
+
+            return $this->fallbackBus;
+        }
+
+        $busName = $busNameStamp->getBusName();
 
         if (!$this->busLocator->has($busName)) {
             throw new InvalidArgumentException(sprintf('Bus named "%s" does not exist.', $busName));
         }
 
-        return $this->busLocator->get($busName)->dispatch($envelope, $stamps);
+        return $this->busLocator->get($busName);
     }
 }
diff --git a/src/Symfony/Component/Messenger/Tests/RoutableMessageBusTest.php b/src/Symfony/Component/Messenger/Tests/RoutableMessageBusTest.php
index c778fd7cbee1..ef970d3838c7 100644
--- a/src/Symfony/Component/Messenger/Tests/RoutableMessageBusTest.php
+++ b/src/Symfony/Component/Messenger/Tests/RoutableMessageBusTest.php
@@ -50,41 +50,34 @@ public function testItRoutesToDefaultBus()
             ->willReturn($envelope);
 
         $container = $this->createMock(ContainerInterface::class);
-        $container->expects($this->once())->method('has')->with(MessageBusInterface::class)
-            ->willReturn(true);
-        $container->expects($this->once())->method('get')->with(MessageBusInterface::class)
-            ->willReturn($defaultBus);
 
-        $routableBus = new RoutableMessageBus($container);
+        $routableBus = new RoutableMessageBus($container, $defaultBus);
 
         $this->assertSame($envelope, $routableBus->dispatch($envelope, [$stamp]));
     }
 
-    public function testItExceptionOnDefaultBusNotFound()
+    public function testItExceptionOnBusNotFound()
     {
         $this->expectException(InvalidArgumentException::class);
-        $this->expectExceptionMessage(sprintf('Bus named "%s" does not exist.', MessageBusInterface::class));
+        $this->expectExceptionMessage('Bus named "my_cool_bus" does not exist.');
 
-        $envelope = new Envelope(new \stdClass());
+        $envelope = new Envelope(new \stdClass(), [
+            new BusNameStamp('my_cool_bus'),
+        ]);
 
         $container = $this->createMock(ContainerInterface::class);
-        $container->expects($this->once())->method('has')->with(MessageBusInterface::class)
-            ->willReturn(false);
-
         $routableBus = new RoutableMessageBus($container);
         $routableBus->dispatch($envelope);
     }
 
-    public function testItExceptionOnBusNotFound()
+    public function testItExceptionOnDefaultBusNotFound()
     {
         $this->expectException(InvalidArgumentException::class);
-        $this->expectExceptionMessage(sprintf('Bus named "%s" does not exist.', 'foo_bus'));
+        $this->expectExceptionMessage('Envelope is missing a BusNameStamp and no fallback message bus is configured on RoutableMessageBus.');
 
-        $envelope = new Envelope(new \stdClass(), [new BusNameStamp('foo_bus')]);
+        $envelope = new Envelope(new \stdClass());
 
         $container = $this->createMock(ContainerInterface::class);
-        $container->expects($this->once())->method('has')->willReturn(false);
-
         $routableBus = new RoutableMessageBus($container);
         $routableBus->dispatch($envelope);
     }