Trying to add CSS classes to form_for

_form.html.haml

= form_for (@user, :html => {:class => "well"}) do |f|
  -if @user.errors.any?
    #error_explanation
      %h2= "#{pluralize(@user.errors.count, "error")} prohibited this user from being saved:"
      %ul
        - @user.errors.full_messages.each do |msg|
          %li= msg

  .field
    = f.label :fname
    = f.text_field :fname
  .field
    = f.label :lname
    = f.text_field :lname
  .field
    = f.label :email
    = f.text_field :email
  .field
    = f.label :password
    = f.text_field :password
  .actions
    = f.submit 'Save'