Cannot return null for non-nullable field Subscription.count.

[m3talstorm]

Describe the Bug

Example taken from https://strawberry.rocks/docs/general/subscriptions + integration with fastapi and uvicorn

import asyncio
from typing import AsyncGenerator

from fastapi import FastAPI, Depends, Request, Response
import strawberry
from strawberry.fastapi import BaseContext, GraphQLRouter
from strawberry.subscriptions import GRAPHQL_TRANSPORT_WS_PROTOCOL, GRAPHQL_WS_PROTOCOL

@strawberry.type
class Query:
    @strawberry.field
    def hello(self) -> str:
        return "world"

@strawberry.type
class Subscription:
    @strawberry.subscription
    async def count(self, target: int = 100) -> AsyncGenerator[int, None]:
        for i in range(target):
            yield i
            await asyncio.sleep(0.5)

schema = strawberry.Schema(query=Query, subscription=Subscription)

graphql_app = GraphQLRouter(
    schema,
    subscription_protocols=[
        GRAPHQL_TRANSPORT_WS_PROTOCOL,
        GRAPHQL_WS_PROTOCOL,
    ],
)

app = FastAPI()
app.include_router(graphql_app, prefix="/graphql")

Running via (in a Docker container): uvicorn app:app --host 0.0.0.0 --port 8080

Getting the following exception:

Cannot return null for non-nullable field Subscription.count.

GraphQL request:2:3
1 | subscription {
2 |   count(target: 5)
  |   ^
3 | }
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/graphql/execution/execute.py", line 540, in execute_field
    completed = self.complete_value(
                ^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/graphql/execution/execute.py", line 620, in complete_value
    raise TypeError(
TypeError: Cannot return null for non-nullable field Subscription.count.

System Information

• Docker image based on python:3.11-slim-bullseye

        'fastapi==0.101.1',
        'uvicorn[standard]==0.23.2',
        'strawberry-graphql[fastapi]==0.203.3'

Additional Context

Upvote & Fund

• We're using Polar.sh so you can upvote and help fund this issue.
• We receive the funding once the issue is completed & confirmed by you.
• Thank you in advance for helping prioritize & fund our backlog.

[m3talstorm]

Still seeing this even with the most simple example.

[patrick91]

from the chat on discord, this seems to be happening when the subscription is being done without using websockets