diff --git a/app/controllers/comments_controller.rb b/app/controllers/comments_controller.rb
index 076eedc..a7f3bf6 100644
--- a/app/controllers/comments_controller.rb
+++ b/app/controllers/comments_controller.rb
@@ -21,14 +21,12 @@ class CommentsController < ApplicationController
       if pc = Comment.find_by_story_id_and_short_id(story.id,
       params[:parent_comment_short_id])
         comment.parent_comment_id = pc.id
-        comment.parent_comment_short_id = pc.short_id
-        comment.thread_id = pc.thread_id
+        # needed for carryng along in comment preview form
+        comment.parent_comment_short_id = params[:parent_comment_short_id]
       else
         return render :json => { :error => "invalid parent comment",
           :status => 400 }
       end
-    else
-      comment.thread_id = Keystore.incremented_value_for("thread_id")
     end
 
     # prevent double-clicks of the post button
diff --git a/app/models/comment.rb b/app/models/comment.rb
index 20d1312..13703c7 100644
--- a/app/models/comment.rb
+++ b/app/models/comment.rb
@@ -13,7 +13,8 @@ class Comment < ActiveRecord::Base
   attr_accessor :parent_comment_short_id, :current_vote, :previewing,
     :indent_level, :highlighted
 
-  before_create :assign_short_id_and_upvote, :assign_initial_confidence
+  before_create :assign_short_id_and_upvote, :assign_initial_confidence,
+    :assign_thread_id
   after_create :assign_votes, :mark_submitter, :deliver_reply_notifications,
     :deliver_mention_notifications
   after_destroy :unassign_votes
@@ -254,6 +255,14 @@ class Comment < ActiveRecord::Base
     self.confidence = self.calculated_confidence
   end
 
+  def assign_thread_id
+    if self.parent_comment_id.present?
+      self.thread_id = self.parent_comment.thread_id
+    else
+      self.thread_id = Keystore.incremented_value_for("thread_id")
+    end
+  end
+
   def unassign_votes
     self.story.update_comment_count!
   end