Rails responding to different formats with AJAX request

I'm trying to understand AJAX requests in Rails. I have a form that I currently submit using remote: true. I want to respond with an HTML redirect if the request is successful, and run an error message with Javascript if it is unsuccessful. However, no matter what the outcome is, the request seems to expect a .html as the return.

respond_to do |format|
  if conversation
    format.html { redirect_to(conversation_path(conversation)) }
  else
    format.js
  end
end

This is called after I save the conversation call on AJAX. On a successful save, the path is correctly rendered. But on an unsuccessful save, it expects the .html and throws an error. How do I accept .js as a response? My goal is to just pop up an error if the call is unsuccessful

Edit: My form_fro:

            <%= form_for :conversation, url: :conversations, html: { class: "conversation-form" } do |f| %>