Error "undefined method 'stringify_keys' for "tag1":String" on multi-model form submission

I'm following the tutorial on http://guides.rubyonrails.org/getting_started.html,
and got stuck on the very last section, "Building a Multi-Model Form".
I'm trying to do two models in the same form, and I get the following
error:
    undefined method `stringify_keys' for "tag1":String

Here is the partial view I am using:

<% form_for(@post) do |f| %>
  <%= f.error_messages %>

  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>
  <p>
    <%= f.label :title %><br />
    <%= f.text_field :title %>
  </p>
  <p>
    <%= f.label :content %><br />
    <%= f.text_area :content %>
  </p>

<h2>Tags</h2>
<% f.fields_for :tags_attributes do |tag_form| %>
<p>
  <%= tag_form.label :name, 'Tag:' %>
  <%= tag_form.text_field :name %>
</p>
<% unless tag_form.object.nil? || tag_form.object.new_record? %>
<p>
  <%= tag_form.label :_delete, 'Remove:' %>
  <%= tag_form.check_box :_delete %>
</p>
<% end %>
<% end %>

  <p>
    <%= f.submit 'Update' %>
  </p>
  <p>
  This was rendered by a partial view!
  </p>
<% end %>

I'm still a beginner, so I hope this question isn't too dumb, but I'm
scratching my head here - any help?