undefined method `stringify_keys!' ruby on rails

I have this code:

def addcar
  @car = Car.new(params[:car])
  render :action => 'list'
end

<% @allcars.each do |cell| %>
  <p>
    <%= link_to cell.to_s, :controller => 'car', :action => 'addcar', :car => cell.to_s %>
  </p>
<% end %>

It's giving me this error:

undefined method `stringify_keys!' for "Honda":String

I don't understand what is wrong with :car.