This has caused me half a day of head scratching.
The default subscription query, as provided by the explorer in GraphiQL will create something like

subscription MySubscription {
  __typename
  count
}

When executed, it results in the server repeatedly failing.

[2022-01-26 18:07:51]: No operation name
subscription MySubscription {
  __typename
  count
}

ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\uvicorn\protocols\websockets\websockets_impl.py", line 190, in run_asgi
    result = await self.app(self.scope, self.asgi_receive, self.asgi_send)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\uvicorn\middleware\proxy_headers.py", line 75, in __call__
    return await self.app(scope, receive, send)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\starlette\applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\starlette\middleware\errors.py", line 146, in __call__
    await self.app(scope, receive, send)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\starlette\middleware\cors.py", line 76, 
in __call__
    await self.app(scope, receive, send)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\starlette\exceptions.py", line 58, in __call__
    await self.app(scope, receive, send)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\starlette\routing.py", line 656, in __call__
    await route.handle(scope, receive, send)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\starlette\routing.py", line 315, in handle
    await self.app(scope, receive, send)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\strawberry\asgi\__init__.py", line 68, in __call__
    await self.graphql_ws_handler_class(
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\strawberry\subscriptions\protocols\graphql_ws\handlers.py", line 66, in handle
    return await self.handle_request()
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\strawberry\asgi\handlers\graphql_ws_handler.py", line 52, in handle_request
    await self.handle_message(message)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\strawberry\subscriptions\protocols\graphql_ws\handlers.py", line 79, in handle_message
    await self.handle_start(message)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\strawberry\subscriptions\protocols\graphql_ws\handlers.py", line 108, in handle_start
    result_source = await self.schema.subscribe(
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\strawberry\schema\schema.py", line 194, 
in subscribe
    return await subscribe(
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\graphql\subscription\subscribe.py", line 58, in subscribe
    result_or_stream = await create_source_event_stream(
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\graphql\subscription\subscribe.py", line 147, in create_source_event_stream
    return await execute_subscription(context)
  File "C:\Users\kristjan\AppData\Local\pypoetry\Cache\virtualenvs\graphql-test-FOqRLQSY-py3.9\lib\site-packages\graphql\subscription\subscribe.py", line 194, in execute_subscription
    raise TypeError(
TypeError: Subscription field must return AsyncIterable. Received: None.