We use push notifications on the site. The worker code is pretty straightforward.

const subscribe = async () => {
    let sub = await self.registration.pushManager.getSubscription();
    if(!sub) {
        sub = await self.registration.pushManager.subscribe({
            applicationServerKey: serverKey(),
            userVisibleOnly: true
        });
    }
    let resp;
    try {
        resp = await fetch('/subscribe', {
            method: 'post',
            headers: {
                'Content-Type': 'application/json'
            },
            body: JSON.stringify(sub)
        });
        const body = await resp.json();
        if(body.status !== 'ok') resp = null;
    } catch(e) {
        resp = null;
    }
    if(resp) return;
    if(sub.unsubscribe) {
        sub.unsubscribe();
    } else {
        self.registration.pushManager.unregister(sub.endpoint);
    }
    await self.registration.unregister();
};

The code above is processed by Babel before landing on the production of cause. It works pretty well for us and for the majority of our users.

But from time to time we receive error reports from the back-end saying the just received subscription data is 'null'. I mean literally: the '/subscribe' endpoint gets 'null' string instead of a JSON encoded subscription object.

Is it possible for pushManager.subscribe to return NULL instead of the subscription object?