Monthly Unique Visitors empty in API response

Hello,

I'm trying to extract daily and monthly unique visitors using the Analytics API, but the metrics are always returning 0 instead of the value seen in the Reports UI. The metric identifiers I'm using are metrics/visitorsmonthly and metrics/visitorsdaily.

An example JSON request:

{
    "rsid": "***",
    "globalFilters": [
        {
            "type": "dateRange",
            "dateRange": "2019-09-16T00:00:00/2019-09-17T00:00:00"
        }
    ],
    "metricContainer": {
        "metrics": [
            {
                "columnId": "pageviews",
                "id": "metrics/pageviews"
            },
            {
                "columnId": "videostart",
                "id": "metrics/videostart"
            },
            {
                "columnId": "timespentvisit",
                "id": "metrics/timespentvisit"
            },
            {
                "columnId": "visitorsmonthly",
                "id": "metrics/visitorsmonthly"
            },
            {
                "columnId": "visitorsdaily",
                "id": "metrics/visitorsdaily"
            }
        ]
    },
    "dimension": "variables/daterangeday",
    "settings": {
        "countRepeatInstances": true,
        "limit": 50,
        "page": 0,
        "dimensionSort": "asc"
    },
    "statistics": {
        "functions": [
            "col-max",
            "col-min"
        ]
    }
}

And the response:

{
  "totalPages": 1,
  "firstPage": true,
  "lastPage": true,
  "numberOfElements": 1,
  "number": 0,
  "totalElements": 1,
  "columns": {
    "dimension": {
      "id": "variables/daterangeday",
      "type": "time"
    },
    "columnIds": [
      "pageviews",
      "videostart",
      "timespentvisit",
      "visitorsmonthly",
      "visitorsdaily"
    ]
  },
  "rows": [
    {
      "itemId": "1190816",
      "value": "Sep 16, 2019",
      "data": [
        652710,
        685738,
        611.7979478807963,
        0,
        0
      ]
    }
  ],
  "summaryData": {
    "nonesFilteredTotals": [
      652710,
      685738,
      611.7979478807963,
      0,
      0
    ],
    "totals": [
      652710,
      685738,
      611.7979478807963,
      0,
      0
    ],
    "col-max": [
      652710,
      685738,
      611.7979478807963,
      0,
      0
    ],
    "col-min": [
      652710,
      685738,
      611.7979478807963,
      0,
      0
    ]
  }
}

Has anyone successfully used these metrics in the Analytics API? Am I missing a requirement for using them?

Thanks!