Rails hidden field undefined method 'merge' error

I wanna do something like this in rails

Here is what I have so far in rails:

<%= form_for @order do |f| %>
  <%= f.hidden_field :service, "test" %>
  <%= f.submit %>
<% end %>

But then I get this error:

undefined method `merge' for "test":String

How can I pass values in my hidden_field in rails?