How can you have a hidden field with simple form?

The following code:

= simple_form_for @movie do |f|
  = f.hidden :title, "some value"
  = f.button :submit

results in this error:

undefined method `hidden' for #SimpleForm::FormBuilder:0x000001042b7cd0
ruby-on-rails ruby-on-rails-3 form-for hidden-field simple-form
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