Capybara/Formulaic doesn't find input as: :radio_button

[Rogerio]

I have an simple_form input that maps to a string attribute, since I want the user to select from fixed options I'm using it as radio_buttons:

<%= s.input :sex, collection: [['M','Male'] ,['F', 'Female']], as: :radio_buttons, label_method: :last, value_method: :first, item_wrapper_class: 'inline', checked: true %>

In my features I have:

fill_form(:student, { name: 'Joe', sex: 'M' })

But capybara isnt beeing able to find it (actually formulaic is not finding it):

 **Formulaic::InputNotFound:**
   Unable to find input "Sex".

The only way it found it was hard referencing it:

        fill_form(:student, {
            name: 'Joe',
            'enrollment[student_attributes][sex]' => 'M'
        })

[calebhearth]

What HTML is being generated from the helper there?