Receiving 500 error when creating flow with ab test

I’m receiving a 500 `A server error occurred` when trying to create a simple flow with an A/B test on an email.
Here’s the response
 

{
  "errors": [
    {
      "id": "2e63cd65-34e9-4885-8301-7865b80b9990",
      "status": 500,
      "code": "error",
      "title": "A server error occurred.",
      "detail": "A server error occurred.",
      "source": {
        "pointer": "/data/"
      }
    }
  ]
}

I obtained my input by creating a flow using the UI editor (to ensure the metrics and templates exist).
I used this as starting point to form the data as per the https://developers.klaviyo.com/en/reference/create_flow API spec.
It should be noted that whilst some of the properties (e.g. cc_email) say they must be a string, the definition of the existing flow has this property as null.

Here’s full API request to the create flows API.
 

{
  "data": {
    "type": "flow",
    "attributes": {
      "name": "My test flow",
      "definition": {
        "triggers": [
          {
            "type": "metric",
            "id": "WwaLQR",
            "trigger_filter": null
          }
        ],
        "profile_filter": null,
        "actions": [
          {
            "temporary_id": "84231258",
            "type": "ab-test",
            "data": {
              "main_action": {
                "temporary_id": "84231212",
                "type": "send-email",
                "data": {
                  "message": {
                    "from_email": "hello@email.com",
                    "from_label": "My email",
                    "reply_to_email": null,
                    "cc_email": null,
                    "bcc_email": null,
                    "subject_line": "Email #1 Subject1",
                    "preview_text": "",
                    "template_id": "QQcUiY",
                    "smart_sending_enabled": true,
                    "transactional": false,
                    "add_tracking_params": false,
                    "custom_tracking_params": null,
                    "additional_filters": null,
                    "name": "Email #1"
                  }
                },
                "links": {
                  "next": null
                }
              },
              "current_experiment": {
                "name": null,
                "variations": [
                  {
                    "temporary_id": "84231433",
                    "type": "send-email",
                    "data": {
                      "message": {
                        "from_email": "hello@email.com",
                        "from_label": "My email",
                        "reply_to_email": null,
                        "cc_email": null,
                        "bcc_email": null,
                        "subject_line": "Email #1 Subject - 1",
                        "preview_text": "",
                        "template_id": "TVh9fA",
                        "smart_sending_enabled": true,
                        "transactional": false,
                        "add_tracking_params": false,
                        "custom_tracking_params": null,
                        "additional_filters": null,
                        "name": "Email #1 Test #2 July 03, 2025 Variation A"
                      }
                    },
                    "links": {
                      "next": null
                    }
                  },
                  {
                    "temporary_id": "84231440",
                    "type": "send-email",
                    "data": {
                      "message": {
                        "from_email": "hello@email.com",
                        "from_label": "My email",
                        "reply_to_email": null,
                        "cc_email": null,
                        "bcc_email": null,
                        "subject_line": "Email #1 Subject - 2",
                        "preview_text": "",
                        "template_id": "RbjbJU",
                        "smart_sending_enabled": true,
                        "transactional": false,
                        "add_tracking_params": false,
                        "custom_tracking_params": null,
                        "additional_filters": null,
                        "name": "Email #1 Test #2 July 03, 2025 Variation B"
                      }
                    },
                    "links": {
                      "next": null
                    }
                  }
                ],
                "allocations": {
                  "84231433": 0.5,
                  "84231440": 0.5
                },
                "winner_metric": "unique-clicks",
                "automatic_winner_selection_settings": {
                  "enabled": true,
                  "automatic_end_date": null,
                  "automatic_end_statistical_certainty": true
                }
              }
            },
            "links": {
              "next": null
            }
          }
        ],
        "entry_action_id": "84231258"
      }
    }
  }
}

Any insights on what might be wrong here?
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