Unable to Subscribe to fulfillments/create and fulfillments/update webhook

[chi-boom]

I wasn't able to subscribe to the fulfillments/create and fulfillments/update webhooks.

Here's my query:

mutation {
  webhookSubscriptionCreate(topic: FULFILLMENTS_CREATE, webhookSubscription: {
    callbackUrl: "callbackUrl"
  }) {
    userErrors {
      field
      message
    }
    webhookSubscription {
      id
    }
  }

I'm getting this response:

Array
(
    [data] => Array
        (
            [webhookSubscriptionCreate] => Array
                (
                    [userErrors] => Array
                        (
                            [0] => Array
                                (
                                    [field] => Array
                                        (
                                            [0] => webhookSubscription
                                        )

                                    [message] => You cannot create a webhook subscription with the specified topic
                                )

                        )

                    [webhookSubscription] => 
                )

        )

    [extensions] => Array
        (
            [cost] => Array
                (
                    [requestedQueryCost] => 10
                    [actualQueryCost] => 10
                    [throttleStatus] => Array
                        (
                            [maximumAvailable] => 1000
                            [currentlyAvailable] => 990
                            [restoreRate] => 50
                        )

                )

        )

)

This webhook was working before until this error appeared. I double checked the topic string, and it seems to be correct. Was there some change in the API that caused this to break? Any advice would be appreciated. Thanks!