Data Index error when querying field ProcessInstances for serviceUrl sub-field

{ ProcessInstances {
  id,
  processId,
  processName,
  state,
  nodes {
    name,
    type,
    enter,
    exit
  },
  serviceUrl
} }

{
  "errors": [
    {
      "message": "Exception while fetching data (/ProcessInstances[0]/serviceUrl) : begin 0, end -1, length 6",
      "locations": [
        {
          "line": 14,
          "column": 5
        }
      ],
      "path": [
        "ProcessInstances",
        0,
        "serviceUrl"
      ],
      "extensions": {
        "classification": "DataFetchingException"
      }
    }
  ],
  "data": {
    "ProcessInstances": [
      {
        "id": "3851d904-f9c1-4767-b6f9-05dcb1dce7ec",
        "processId": "demo.orders",
        "processName": "orders",
        "parentProcessInstanceId": null,
        "rootProcessInstanceId": null,
        "roles": [],
        "state": "ACTIVE",
        "start": "2020-04-07T06:20:52.862Z",
        "lastUpdate": "2020-04-07T06:20:52.889Z",
        "addons": [
          "process-management"
        ],
        "businessKey": null,
        "serviceUrl": null,
        "error": null,
        "__typename": "ProcessInstance"
      }
    ]
  }
}