为什么我会得到这个未初始化的常量错误?

这是我得到的错误: uninitialized constant SellController ,当我尝试转到此URL时: http://localhost:3000/port_stocks/6/sell/new

这是我的路线:

  resources :port_stocks do
    resources :sell
  end

  namespace :port_stocks do
    resources :buy
  end

这是我的 app/controllers/port_stocks/sell_controller.rb

class SellController < ApplicationController

  def show
  end

  def create
    # Lots of code here
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_port_stock
      @port_stock = PortStock.find(params[:id])
    end

    def set_portfolio
      @portfolio = current_user.portfolio
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def port_stock_params
      params.require(:port_stock).permit(:portfolio_id, :stock_id, :volume, :purchase_price, :current_price, :percent_change, :bought_on, :action)
    end
end

这是什么原因?