Back In Stock Subscription with new API fails

I’m building a custom Back in Stock UX for a Shopify store using the latest Klaviyo API and I followed the JavaScript example in the docs @ https://developers.klaviyo.com/en/reference/create_client_back_in_stock_subscription.

Here is my fetch() call:

const options = {
  method: 'POST',
  headers: {
    accept: 'application/json',
    revision: '2023-08-15',
    'content-type': 'application/json'
  },
  body: JSON.stringify({
    data: {
      type: 'back-in-stock-subscription',
      attributes: {
        channels: ['EMAIL', 'SMS'],
        profile: {
          data: {
            type: 'profile',
            attributes: {
              email: 'fetch.test@klaviyo-demo.com',
              phone_number: '+15005550006'
            }
          },
          email: 'fetch.test@klaviyo-demo.com',
          phone_number: '+15005550006'
        }
      },
      relationships: {
        variant: {
          data: {
            type: 'catalog-variant',
            id: '$shopify:::$default:::39986281939057'
          }
        }
      }
    }
  })
};

fetch('https://a.klaviyo.com/client/back-in-stock-subscriptions/?company_id=PUBLIC_API_KEY', options)
  .then(response => response.json())
  .then(response => console.log(response))
  .catch(err => console.error(err));

The result is a console error: SyntaxError: Unexpected end of JSON input

I do see that in my dev tools network tab that the response code is 202, which the documentation indicates as a successful request, however I do not see the profile get added to my client’s list of profiles (I did test with the old/deprecated API and that worked fine).

In the first then() of my fetch() call, if I do a quick console.log(response) before returning the .json() promise, I can actually see the response is JSON and the response.ok property is true, but no matter what I try, the profile does not get added to my client’s list of profiles. I also tried this using the cURL example and that worked right away, saw the profile show up almost immediately.

Any help on this would be much appreciated!