diff --git a/app/assets/javascripts/discourse/models/composer.js.es6 b/app/assets/javascripts/discourse/models/composer.js.es6
index 1d5961bcd17..366a55e73e9 100644
--- a/app/assets/javascripts/discourse/models/composer.js.es6
+++ b/app/assets/javascripts/discourse/models/composer.js.es6
@@ -431,6 +431,8 @@ const Composer = RestModel.extend({
       if (!this.get('topic')) {
         this.set('topic', opts.post.get('topic'));
       }
+    } else {
+      this.set('post', null);
     }
 
     this.setProperties({
diff --git a/lib/post_creator.rb b/lib/post_creator.rb
index faf713fbe5f..182cb0a3660 100644
--- a/lib/post_creator.rb
+++ b/lib/post_creator.rb
@@ -56,6 +56,8 @@ class PostCreator
     @opts = opts || {}
     opts[:title] = pg_clean_up(opts[:title]) if opts[:title] && opts[:title].include?("\u0000")
     opts[:raw] = pg_clean_up(opts[:raw]) if opts[:raw] && opts[:raw].include?("\u0000")
+    opts.delete(:reply_to_post_number) unless opts[:topic_id]
+
     @spam = false
   end
 
diff --git a/spec/components/post_creator_spec.rb b/spec/components/post_creator_spec.rb
index 0595a66e007..8ec10e457b3 100644
--- a/spec/components/post_creator_spec.rb
+++ b/spec/components/post_creator_spec.rb
@@ -38,27 +38,29 @@ describe PostCreator do
       expect { creator.create }.to raise_error(Discourse::InvalidAccess)
     end
 
+    context "reply to post number" do
+      it "omits reply to post number if received on a new topic" do
+        p = PostCreator.new(user, basic_topic_params.merge(reply_to_post_number: 3)).create
+        expect(p.reply_to_post_number).to be_nil
+      end
+    end
 
     context "invalid title" do
-
       let(:creator_invalid_title) { PostCreator.new(user, basic_topic_params.merge(title: 'a')) }
 
       it "has errors" do
         creator_invalid_title.create
         expect(creator_invalid_title.errors).to be_present
       end
-
     end
 
     context "invalid raw" do
-
       let(:creator_invalid_raw) { PostCreator.new(user, basic_topic_params.merge(raw: '')) }
 
       it "has errors" do
         creator_invalid_raw.create
         expect(creator_invalid_raw.errors).to be_present
       end
-
     end
 
     context "success" do
diff --git a/spec/controllers/posts_controller_spec.rb b/spec/controllers/posts_controller_spec.rb
index 76e76f6f3da..748cfca7c06 100644
--- a/spec/controllers/posts_controller_spec.rb
+++ b/spec/controllers/posts_controller_spec.rb
@@ -618,7 +618,7 @@ describe PostsController do
         end
 
         it "passes reply_to_post_number through" do
-          xhr :post, :create, {raw: 'hello', reply_to_post_number: 6789}
+          xhr :post, :create, {raw: 'hello', reply_to_post_number: 6789, topic_id: 1234}
           expect(assigns(:manager_params)['reply_to_post_number']).to eq('6789')
         end
 
@@ -678,7 +678,7 @@ describe PostsController do
       end
 
       it "ensures regular user cannot see the revisions" do
-        u = log_in(:user)
+        log_in(:user)
         xhr :get, :revisions, post_id: post_revision.post_id, revision: post_revision.number
         expect(response).to be_forbidden
       end
@@ -831,7 +831,7 @@ describe PostsController do
       it "doesn't return secured categories for moderators if they don't have access" do
         user = Fabricate(:user)
         admin = Fabricate(:admin)
-        moderator = Fabricate(:moderator)
+        Fabricate(:moderator)
 
         group = Fabricate(:group)
         group.add(user)
@@ -852,7 +852,7 @@ describe PostsController do
       it "doesn't return PMs for moderators" do
         user = Fabricate(:user)
         admin = Fabricate(:admin)
-        moderator = Fabricate(:moderator)
+        Fabricate(:moderator)
 
         pm_post = create_post(user: user, archetype: 'private_message', target_usernames: [admin.username])
         PostDestroyer.new(admin, pm_post).destroy
@@ -869,7 +869,7 @@ describe PostsController do
         user = Fabricate(:user)
         admin = Fabricate(:admin)
 
-        post_not_deleted = create_post(user: user)
+        create_post(user: user)
         post_deleted_by_user = create_post(user: user)
         post_deleted_by_admin = create_post(user: user)