Console error with Klaviyo JS (subscribe_profiles)

Question

I have a hook in my Next JS project, which subscribes a user to a Klaviyo list when they submit the form on the site.

The profile is being added to Klaviyo just fine, and it all works okay from the user's perspective.

However I am getting the following console error:

klaviyoSignup.js:37 Fetch Error: SyntaxError: Unexpected end of JSON input at eval (klaviyoSignup.js:34:23)

The response returned from Klaviyo says:

{
   "errors": [
      {
         "id": "XXXXXX",
         "status": 405,
         "code": "method_not_allowed",
         "title": "Method \"{method}\" not allowed.",
         "detail": "Method \"list\" not allowed.",
         "source": {
         "pointer": "/data/"
      }
   ]
}

Here is the code for the hook:

export function klaviyoSignup(emailValue) {
  console.log(emailValue);

  // https://developers.klaviyo.com/en/reference/subscribe_profiles

  const options = {
    method: 'POST',
    headers: { 'revision': '2023-12-15', 'content-type': 'application/json' },
    body: JSON.stringify({
      data: {
        type: 'subscription',
        attributes: {
          profile: {
            data: {
              type: 'profile',
              attributes: {
                email: emailValue
              }
            }
          }
        },
        relationships: { list: { data: { type: 'list', id: 'XXXXXX' } } }
      }
    })
  };

  fetch('https://a.klaviyo.com/client/subscriptions/?company_id=XXXXXX', options)
    .then(response => {
      console.log('Response Status:', response.status);
      console.log('Response Headers:', response.headers);

      if (!response.ok) {
        throw new Error(`HTTP error! Status: ${response.status}`);
      }

      return response.json();
    })
    .then(response => console.log('Response JSON:', response))
    .catch(err => console.error('Fetch Error:', err));
}

This is using code slightly modified from Klaviyo's docs (but the functionality is all the same as their example):

https://developers.klaviyo.com/en/reference/subscribe_profiles