diff --git a/lib/pleroma/web/activity_pub/transmogrifier.ex b/lib/pleroma/web/activity_pub/transmogrifier.ex
index 98a2af819..26b2dd575 100644
--- a/lib/pleroma/web/activity_pub/transmogrifier.ex
+++ b/lib/pleroma/web/activity_pub/transmogrifier.ex
@@ -649,7 +649,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     if object = Object.normalize(id), do: {:ok, object}, else: nil
   end
 
-  def set_reply_to_uri(%{"inReplyTo" => inReplyTo} = object) do
+  def set_reply_to_uri(%{"inReplyTo" => inReplyTo} = object) when is_binary(inReplyTo) do
     with false <- String.starts_with?(inReplyTo, "http"),
          {:ok, %{data: replied_to_object}} <- get_obj_helper(inReplyTo) do
       Map.put(object, "inReplyTo", replied_to_object["external_url"] || inReplyTo)
@@ -765,12 +765,18 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
   def add_hashtags(object) do
     tags =
       (object["tag"] || [])
-      |> Enum.map(fn tag ->
-        %{
-          "href" => Pleroma.Web.Endpoint.url() <> "/tags/#{tag}",
-          "name" => "##{tag}",
-          "type" => "Hashtag"
-        }
+      |> Enum.map(fn
+        # Expand internal representation tags into AS2 tags.
+        tag when is_binary(tag) ->
+          %{
+            "href" => Pleroma.Web.Endpoint.url() <> "/tags/#{tag}",
+            "name" => "##{tag}",
+            "type" => "Hashtag"
+          }
+
+        # Do not process tags which are already AS2 tag objects.
+        tag when is_map(tag) ->
+          tag
       end)
 
     object
diff --git a/test/web/activity_pub/transmogrifier_test.exs b/test/web/activity_pub/transmogrifier_test.exs
index e5e3c8d33..86c66deff 100644
--- a/test/web/activity_pub/transmogrifier_test.exs
+++ b/test/web/activity_pub/transmogrifier_test.exs
@@ -1128,4 +1128,58 @@ defmodule Pleroma.Web.ActivityPub.TransmogrifierTest do
         )
     end
   end
+
+  describe "reserialization" do
+    test "successfully reserializes a message with inReplyTo == nil" do
+      user = insert(:user)
+
+      message = %{
+        "@context" => "https://www.w3.org/ns/activitystreams",
+        "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+        "cc" => [],
+        "type" => "Create",
+        "object" => %{
+          "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+          "cc" => [],
+          "type" => "Note",
+          "content" => "Hi",
+          "inReplyTo" => nil,
+          "attributedTo" => user.ap_id
+        },
+        "actor" => user.ap_id
+      }
+
+      {:ok, activity} = Transmogrifier.handle_incoming(message)
+
+      {:ok, _} = Transmogrifier.prepare_outgoing(activity.data)
+    end
+
+    test "successfully reserializes a message with AS2 objects in IR" do
+      user = insert(:user)
+
+      message = %{
+        "@context" => "https://www.w3.org/ns/activitystreams",
+        "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+        "cc" => [],
+        "type" => "Create",
+        "object" => %{
+          "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+          "cc" => [],
+          "type" => "Note",
+          "content" => "Hi",
+          "inReplyTo" => nil,
+          "attributedTo" => user.ap_id,
+          "tag" => [
+            %{"name" => "#2hu", "href" => "http://example.com/2hu", "type" => "Hashtag"},
+            %{"name" => "Bob", "href" => "http://example.com/bob", "type" => "Mention"}
+          ]
+        },
+        "actor" => user.ap_id
+      }
+
+      {:ok, activity} = Transmogrifier.handle_incoming(message)
+
+      {:ok, _} = Transmogrifier.prepare_outgoing(activity.data)
+    end
+  end
 end