core: HTML5 notifications not working in 2021.4.0b5

The problem

HTML5 notifications have stopped working since upgrading to 2021.4.0b5.

What is version of Home Assistant Core has the issue?

2021.4.0b5

What was the last working version of Home Assistant Core?

2021.4.0b4

What type of installation are you running?

Home Assistant OS

Integration causing the issue

html5

Link to integration documentation on our website

https://www.home-assistant.io/integrations/html5/

Example YAML snippet

action:
    - service: notify.html5_1oasis
      data:
        title: 1 Oasis
        message: SYSTEM | Server Reboot

Anything in the logs that might be useful for us?

Logger: homeassistant.helpers.script.websocket_api_script
Source: custom_components/html5/notify.py:576
First occurred: 6:51:01 PM (2 occurrences)
Last logged: 6:51:14 PM

websocket_api script: Error executing script. Unexpected error for call_service at pos 1: Missing 'sub' from claims. 'sub' is your admin email as a mailto: link.
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/script.py", line 359, in _async_step
    await getattr(self, handler)()
  File "/usr/src/homeassistant/homeassistant/helpers/script.py", line 559, in _async_call_service_step
    await service_task
  File "/usr/src/homeassistant/homeassistant/core.py", line 1480, in async_call
    task.result()
  File "/usr/src/homeassistant/homeassistant/core.py", line 1515, in _execute_service
    await handler.job.target(service_call)
  File "/usr/src/homeassistant/homeassistant/components/notify/__init__.py", line 160, in _async_notify_message_service
    await self.async_send_message(**kwargs)
  File "/usr/src/homeassistant/homeassistant/components/notify/__init__.py", line 137, in async_send_message
    await self.hass.async_add_executor_job(
  File "/usr/local/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/config/custom_components/html5/notify.py", line 492, in send_message
    self._push_message(payload, **kwargs)
  File "/config/custom_components/html5/notify.py", line 525, in _push_message
    vapid_headers = create_vapid_headers(
  File "/config/custom_components/html5/notify.py", line 576, in create_vapid_headers
    return vapid.sign(vapid_claims)
  File "/usr/local/lib/python3.8/site-packages/py_vapid/__init__.py", line 320, in sign
    sig = sign(self._base_sign(claims), self.private_key)
  File "/usr/local/lib/python3.8/site-packages/py_vapid/__init__.py", line 270, in _base_sign
    raise VapidException(
py_vapid.VapidException: Missing 'sub' from claims. 'sub' is your admin email as a mailto: link.

Have tried on 2 different systems and the errors are consistent across both.