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

I'm following the tutorial on Getting Started with Rails — Ruby on Rails Guides, 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?