title: Nested form returns undefined method `with_indifferent_access'
body: <p>I have a student model and a referee model. I want the student to fill out a form, that has a nested area which requests the information to contact the referee. I'm using activeadmin and formtastic with rails 3.2.6.</p>
  
  <p>My issue is when I navigate to new_student_path, I get "undefined method `with_indifferent_access'"</p>
  
  <p>students_controller.rb:</p>
  
  <pre><code>  def new
     @student = Student.new
     @student.referee = Referee.new
    end
  </code></pre>
  
  <p>student model:</p>
  
  <pre><code>class Student &lt; ActiveRecord::Base
  
  has_one :referee
  accepts_nested_attributes_for :referee
  attr_accessible :firstname, :lastname, :statement
  attr_accessible :referee, :referee_attributes
  alias_method :referee=, :referee_attributes=
  </code></pre>
  
  <p>Student's new.html.erb:</p>
  
  <pre><code>&lt;%= semantic_form_for @student do |f| %&gt;
  &lt;%= f.semantic_errors :state %&gt;
  
   &lt;%= f.inputs do %&gt;
      &lt;%= f.input :firstname %&gt;
      &lt;%= f.input :lastname %&gt;
  
      &lt;%= f.semantic_fields_for :referee do |ff| %&gt;
          &lt;% if h.object.new_record? %&gt;
          &lt;%= ff.inputs do %&gt;
              &lt;%= ff.input :firstname, :label =&gt; "Reference's First Name" %&gt;
              &lt;%= ff.input :lastname, :label =&gt; "Reference's Last Name"%&gt;
              &lt;%= ff.input :email, :label =&gt; "Reference's Email"%&gt;
          &lt;% end %&gt;
        &lt;% end %&gt;
      &lt;% end %
  </code></pre>