Why can't Tornado Websockets see cookies?

Question

Python

Cookies are immediately sent to the client on the main page:

class IndexHandler(RequestHandler):
  def get(self):
    if not self.get_secure_cookie("user"):
      self.set_secure_cookie("user", "1213", domain='127.0.0.1')
    self.render("index.html",)

In index.html there is a js code that is responsible for connecting to the host:

var ws = new WebSocket("ws://localhost:8888/websocket");
// и т.д

Everything works great (messages are sent and delivered), cookies are visible in the browser, but there is a problem - Tornado for some reason does not see cookies:

class HiHandler(WebSocketHandler):
  def open(self):
    print("WebSocket opened")
    print("received cookies: ", self.request.cookies)
    print("received 'myuser': ", self.get_secure_cookie("user"))

The output is:

WebSocket opened
received cookies:  
received 'myuser':  None

Why is this happening and how to fix it?
Thank you!