Apple Search Ads Reports API - Invalid Request JSON

I'm trying to call https://api.searchads.apple.com/api/v5/reports/campaigns but getting the following response:

{"data":null,"pagination":null,"error":{"errors":[{"messageCode":"INVALID_JSON_REQUEST","message":"This is an invalid json. The request can not be parsed","field":"Line#:1 Column#:13"}]}}

I've used all suggested options for request parameters from documentation, e.g.

{
  "startTime": "2024-04-08",
  "endTime": "2024-04-09",
  "selector": {
    "orderBy": [
      {
        "field": "countryOrRegion",
        "sortOrder": "ASCENDING"
      }
    ],
    "conditions": [
      {
        "field": "countriesOrRegions",
        "operator": "CONTAINS_ANY",
        "values": [
          "US",
          "GB"
        ]
      },
      {
        "field": "countryOrRegion",
        "operator": "IN",
        "values": [
          "US"
        ]
      }
    ],
    "pagination": {
      "offset": 0,
      "limit": 1000
    }
  },
  "groupBy": [
    "countryOrRegion"
  ],
  "timeZone": "UTC",
  "returnRecordsWithNoMetrics": true,
  "returnRowTotals": true,
  "returnGrandTotals": true
}

What's the proper way to call this API?