ruby - Virtus and booleans conversion -

i'm using virtus gem , trying autocast strings booleans, without success... if can see what's wrong code...

virtus.coercer |config|   config.string.boolean_map = { 'true' => true, 'false' => false } end  class bookingwizard   include virtus    extend activemodel::naming   include activemodel::conversion   include activemodel::validations    attribute :know_doctor, boolean, default: false end  1.9.3 (main):0 > b = bookingwizard.new => #<bookingwizard:0x007fea748bf338  @know_doctor=false> 1.9.3 (main):0 > b.know_doctor = "true" => "true" 1.9.3 (main):0 > b.know_doctor => "true" 1.9.3 (main):0 > b.know_doctor.class => string 1.9.3 (main):0 > virtus.coercer[string].to_boolean("true") => true