Lua 错误：尝试索引全局变量 'concommand' （值为 nil）。

我想用Lua制作一个简单的网页浏览器。我有以下脚本。但是它不能编译并且出现错误：尝试索引全局变量'concommand'（一个空值）。有人能告诉我我的脚本有什么问题吗？

    --醉人的简单网页浏览器

如果SERVER然后AddCSLuaFile("web_browser.lua")返回end

concommand.Add("OpenWebBrowser", function()

local homepage = "http://www.google.com"

local frame = vgui.Create("DFrame")
frame:SetSize(ScrW()*0.8, ScrH()*0.8)
frame:SetTitle("")
frame:SetDraggable(false)
frame:Center()
frame:MakePopup()
frame.Paint = function()
    surface.SetDrawColor(0, 0, 0, 150)
    surface.DrawRect(0, 0, frame:GetWide(), frame:GetTall())
end

local lbl_loading = vgui.Create("DLabel", frame)
lbl_loading:SetText("加载中. . .")
surface.CreateFont("coolvetica", 64, 400, true, false, "LoadFont")
lbl_loading:SetFont("LoadFont")
lbl_loading:SetColor(Color(255, 255, 255))
lbl_loading:SizeToContents()
lbl_loading:Center()

local html_pos = { 12, 48 }
local html_size = { frame:GetWide()-html_pos[1]*2, frame:GetTall()-html_pos[2]-html_pos[1] }
local html_frame = vgui.Create("HTML", frame)
html_frame:SetPos(html_pos[1], html_pos[2])
html_frame:SetSize(html_size[1], html_size[2])
html_frame:OpenURL(homepage)

local lbl_goto = vgui.Create("DLabel", frame)
lbl_goto:SetText("转到：")
surface.CreateFont("coolvetica", 24, 400, true, false, "GotoFont")
lbl_goto:SetFont("GotoFont")
lbl_goto:SetColor(Color(255, 255, 255))
lbl_goto:SetPos(12, 16)
lbl_goto:SizeToContents()

local txt_goto = vgui.Create("DTextEntry", frame)
txt_goto:SetSize(frame:GetWide()-96, 24)
txt_goto:SetPos(64, 14)
txt_goto:SetText(homepage)
txt_goto.OnEnter = function()
    local site = string.lower(txt_goto:GetValue())
  --      if string.Left( site, 7 ) != "http://" then
  --         site = "http://" .. site
  --      txt_goto:SetText( site )
  --  end
    html_frame:OpenURL(site)
end

end)