Ruby on Rails

I have a model with a boolean variable in the database and one added by attr_accessor:

In the model:

  attr_accessor :attrvalue

  validates(:dbvalue, inclusion: { in: [true, false], message: "%{value} is not a valid response"} )

  validates(:attrvalue, inclusion: { in: [true, false], message: "%{value} is not a valid response"} )

In the view:

    <%= form_for(@user) do |f| %>

      <%= f.select :dbvalue, {'' => nil, 'Yes' => true, 'No' => false}, {},  { :class => 'span1' } %>

      <%= f.select :attrvalue, {'' => nil, 'Yes' => true, 'No' => false}, {},  { :class => 'span1' } %>

    <% end %>

The dbvalue works, the attrvalue does not work and always throws the message attrvalue is not a valid response.

I assume that attrvalue is not a boolean. Can I force it to be a boolean? Alternately, is there another way to handle it?