Mongodb - unknown top level operator: $elemMatch"

I have a collection which contains some documents as follows,

{
  "transactionId": 3432423,
  "reviews": [
    {
      "fromUser": {
        "userId": "eu-central-1:-4e37-ad56-5236aa1acd6e"
      },
      "toUser": {
        "userId": "eu-central-1:8e4934bb-7533-4e0d-9f78-0ec8db9544cc"
      },
      "rating": 4.3,
      "comment": "good listing"
    },
    {
    "toUser": {
        "userId": "eu-central-23e-4e37-ad56-5236aa1acd6e",
        "email": "yourlife@gmail.com",
        "firstName": "your",
        "lastName": "life"
      },
      "fromUser": {
        "userId": "eu-central-133-4e0d-9f78-0ec8db9544cc",
        "email": "mylife@gmail.com",
        "firstName": "my",
        "lastName": "life"
      },
      "rating": 4.3,
      "comment": "bad listing"
    }
  ]

}

i need to check if subdocument reviews present inside the document. i have used this query,

db.getCollection('_testCollection').find({  "$elemMatch": { "reviews": { "$exists": false } }})

it throws an error saying,

"errmsg" : "unknown top level operator: $elemMatch",

via Sajeetharan