old_contents: from __future__ import absolute_import
  
  from django.http import HttpResponse, HttpRequest
  from typing import Any, List, Dict, Optional, Text
  
  from zerver.lib.response import json_error, json_success
  from zerver.lib.user_agent import parse_user_agent
  
  def check_compatibility(request):
      # type: (HttpRequest) -> HttpResponse
      user_agent = parse_user_agent(request.META["HTTP_USER_AGENT"])
      if user_agent['name'] == "ZulipInvalid":
          return json_error("Client is too old")
      return json_success()
  
new_contents: from __future__ import absolute_import
  
  from django.http import HttpResponse, HttpRequest
  from typing import Any, List, Dict, Optional, Text
  
  from zerver.lib.response import json_error, json_success
  from zerver.lib.user_agent import parse_user_agent
  
  def check_compatibility(request):
      # type: (HttpRequest) -> HttpResponse
      user_agent = parse_user_agent(request.META["HTTP_USER_AGENT"])
      if user_agent is None or user_agent['name'] == "ZulipInvalid":
          return json_error("Client is too old")
      return json_success()
  
subject: Add check for parse_user_agent returning None in compatibility.py.