diff --git a/src/ActivityEventHandlers/ActivityPersister.php b/src/ActivityEventHandlers/ActivityPersister.php
index 8e67238..f17b52a 100644
--- a/src/ActivityEventHandlers/ActivityPersister.php
+++ b/src/ActivityEventHandlers/ActivityPersister.php
@@ -3,6 +3,7 @@
 namespace ActivityPub\ActivityEventHandlers;
 
 use ActivityPub\Objects\CollectionsService;
+use ActivityPub\Objects\IdProvider;
 use ActivityPub\Objects\ObjectsService;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
@@ -18,6 +19,11 @@ class ActivityPersister implements EventSubscriberInterface
      */
     private $objectsService;
 
+    /**
+     * @var IdProvider
+     */
+    private $idProvider;
+
     public static function getSubscribedEvents()
     {
         return array(
@@ -27,10 +33,12 @@ class ActivityPersister implements EventSubscriberInterface
     }
 
     public function __construct( CollectionsService $collectionsService,
-                                 ObjectsService $objectsService )
+                                 ObjectsService $objectsService,
+                                 IdProvider $idProvider )
     {
         $this->collectionsService = $collectionsService;
         $this->objectsService = $objectsService;
+        $this->idProvider = $idProvider;
     }
 
     public function persistActivityToInbox( InboxActivityEvent $event )
@@ -47,6 +55,9 @@ class ActivityPersister implements EventSubscriberInterface
     public function persistActivityToOutbox( OutboxActivityEvent $event )
     {
         $activity = $event->getActivity();
+        if ( ! array_key_exists( 'id', $activity ) ) {
+            $activity['id'] = $this->idProvider->getId( $event->getRequest(), "activities" );
+        }
         $receivingActor = $event->getReceivingActor();
         if ( $receivingActor->hasField( 'outbox' ) ) {
             $this->collectionsService->addItem( $receivingActor['outbox'], $activity );
diff --git a/src/Config/ActivityPubModule.php b/src/Config/ActivityPubModule.php
index 160c0ad..c8f0f86 100644
--- a/src/Config/ActivityPubModule.php
+++ b/src/Config/ActivityPubModule.php
@@ -180,7 +180,8 @@ class ActivityPubModule
 
         $this->injector->register( ActivityPersister::class, ActivityPersister::class )
             ->addArgument( new Reference( CollectionsService::class ) )
-            ->addArgument( new Reference( ObjectsService::class ) );
+            ->addArgument( new Reference( ObjectsService::class ) )
+            ->addArgument( new Reference( IdProvider::class ) );
     }
 
     /**