diff --git a/app/serializers/admin_user_action_serializer.rb b/app/serializers/admin_user_action_serializer.rb
index 4dd05564baf..1bff61203db 100644
--- a/app/serializers/admin_user_action_serializer.rb
+++ b/app/serializers/admin_user_action_serializer.rb
@@ -52,15 +52,15 @@ class AdminUserActionSerializer < ApplicationSerializer
   end
 
   def slug
-    object.topic&.slug
+    topic&.slug
   end
 
   def title
-    object.topic&.title
+    topic&.title
   end
 
   def category_id
-    object.topic&.category_id
+    topic&.category_id
   end
 
   def moderator_action
@@ -80,4 +80,14 @@ class AdminUserActionSerializer < ApplicationSerializer
       .select { |ua| [UserAction::REPLY, UserAction::RESPONSE].include? ua.action_type }
       .first.try(:action_type)
   end
+
+  # we need this to handle deleted topics which aren't loaded via
+  # Topic.unscoped do
+  #   Post.includes(:topic)
+  # end
+  # because Rails 4 "unscoped" support is still bugged (cf. https://github.com/rails/rails/issues/13775)
+  def topic
+    return @topic if @topic
+    @topic = object.topic || Topic.with_deleted.find_by_id(object.topic_id)
+  end
 end
diff --git a/spec/serializers/admin_user_action_serializer_spec.rb b/spec/serializers/admin_user_action_serializer_spec.rb
new file mode 100644
index 00000000000..371e76468a7
--- /dev/null
+++ b/spec/serializers/admin_user_action_serializer_spec.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+describe AdminUserActionSerializer do
+  fab!(:user) { Fabricate(:user) }
+  fab!(:admin) { Fabricate(:admin) }
+  let(:guardian) { Guardian.new(admin) }
+
+  fab!(:topic) { Fabricate(:topic) }
+  fab!(:post) { Fabricate(:post, topic: topic) }
+
+  it "includes the slug/title/category ID for a post's deleted topic" do
+    topic.trash!
+
+    json = AdminUserActionSerializer.new(post, scope: guardian, root: false).as_json
+
+    expect(json[:slug]).to eq(topic.slug)
+    expect(json[:title]).to eq(topic.title)
+    expect(json[:category_id]).to eq(topic.category_id)
+  end
+end