Why do hidden fields produce hashes?

Question

My Hidden Field:

- @calc.results.each do |k, v|
  = hidden_field :calc_result, :value => "#{k[:total_interest]}"

Which returns :

"calc_result"=>
{"value214.14"=>"",
...

How can I write the hidden_field so that it produces :

"value" => "214.14"