Why my splash:select always fail in this simple code (Splash)?

Question

I want to select the "2nd-page" button at the end of this website using Splash code (Docker/Browser ver). I even used the "copy Selector" function of Chrome to get to the extract part, I also checked with F12 and CtrlF and found the extract position. But somehow my code still fail at the select function

Below is my simple Lua Splash code:

function main(splash, args)

  splash.private_mode_enabled = false
  url = args.url
  assert(splash:go(url))
  assert(splash:wait(1))
  next_page = assert(splash:select('div > div.py20 > div > nav > ul > li:nth-child(3) > a'))
  assert(splash:wait(1))
  next_page:mouse_click()
  return {splash:html()}
 end

My code always fails at the next_page line with an assertion error:

{
    "error": 400,
    "type": "ScriptError",
    "description": "Error happened while executing Lua script",
    "info": {
        "source": "[string \"function main(splash, args)\r...\"]",
        "line_number": 8,
        "error": "assertion failed!",
        "type": "LUA_ERROR",
        "message": "Lua error: [string \"function main(splash, args)\r...\"]:8: assertion failed!"
    }
}