Reform multiparameter date issue

new.html.erb

<%= simple_form_for @form do |f| %>
  <%= f.input :username %>
  <%= f.input :birthdate, as: :date %>
  <%= f.button :submit %>
<% end %>

user.rb

class User < ActiveRecord::Base
end

user_form.rb

class UserForm < Reform::Form
  include ModelReflections

  property :username
  property :birthdate, multi_params: true, type: Date
end

users_controller.rb

class UsersController < ApplicationController
  def new
    @form = UserForm.new(User.new)
  end

  def create
    @form = UserForm.new(User.new)

    if @form.validate(params[:user])
      @form.save
    end
    redirect_to @form.model
  end

  def show
    @user = User.find(params[:id])
  end
end