Make simple_form require an answer before submitting

I am writing a rails app and using simple form for forms. I have a form that has a user select answers to a question in the form of radio buttons. However, the form currently allows the user to submit an answer without choosing an option, even if I put requires true in the simple_form. Anyone have any idea how to actually require an answer first?

<%= simple_form_for @question do |f| %>
        <%= f.error_notification %>
        <%= f.association :answer, as: :radio_buttons, label_method: :name, required: true %>
        <%= f.button :submit, "Submit" %>
        <% end %>