无法为不可为空的字段 Subscription.counter 返回 null

Question

我已将 https://github.com/apollographql/apollo-server 集成到 NextJS api 中。除了订阅之外，几乎一切都按预期工作。 我设置了一个非常简单的订阅如下：

type Subscription {
    counter: Int!
}

解析器：

const Subscription: SubscriptionResolvers = {
    counter: {
        subscribe: (parent, args, ctx: Services) => {
            let count = 0
            setTimeout(() => {
                count++
                ctx.pubSub.publish("COUNTER", count)
            }, 500);
            return ctx.pubSub.asyncIterator("COUNTER");
        }
    }
}


export {Subscription as default}


export const resolvers: Resolvers = {
    Query: {
        greet: (parent, args) => {
            return `Hello ${args.name}`;
        }
    },
    Subscription: Subscription
};

运行订阅，我有：

我做错了什么？代码托管在 https://github.com/softshipper/next-sub。