Passing identifier in analytics service REST API throws error

curl --location --request POST 'http://10.112.205.101:8095/analytics/service' \ --header 'Content-Type: application/x-www-form-urlencoded' \ --header 'Accept: */*' \ --header 'Authorization: Basic QWRtaW5pc3RyYXRvcjpwYXNzd29yZA==' \ --data-urlencode 'statement=select * from x where ($type is null OR `x`.`type` = $type) AND (`x`.`fname` is null OR `x`.`fname` = "c") AND (`x`.`lname` is null OR `x`.`lname` = "b");' \ --data-urlencode '$type=a'

{ "errors": [ { "code": 21009, "msg": "Unable to process JSON content in request" } ], "status": "fatal", "metrics": { "elapsedTime": "470.28µs", "executionTime": "0ns", "resultCount": 0, "resultSize": 0, "processedObjects": 0, "errorCount": 1 } }