Trying to get a POST to return 400 bad request

I have a create method that builds a new model through an association and I was expecting it to return a 400 response with some text if no params were in the POST request. However, I get an error.

This is in Rails 4.0.2

controller methods:

  def create
    @cast_profile = current_user.build_cast_profile(cast_profile_params)
    if @cast_profile.save
      redirect_to cast_profile_path
    else
      render :edit
    end
  end

  def cast_profile_params
    params.require(:cast_profile).permit(:name, :email, :public)
  end

If I pass the params its all fine but I'm trying to test the bad request scenario. Here's the error:

ActionController::ParameterMissing: param not found: cast_profile

I could rescue it explicitly but I thought strong parameters was supposed to do that automatically.