Java code for mongo aggregate query containing $push

Below is a pipeline snippet for mongo query. And the corresponding Java code that I'm trying to write. I'm getting the error 'Unrecognized expression '$push''. Can someone please let me know what is wrong with the Java code? Other pipelines seem to be fine. Issue is with $push.

I'm using mongo driver 3.8.2 with latest Mongo 5 version

    "$group": {
      "_id": {
        "a": "$_id.a",
        "b": "$_id.b"
      },
      "data": {
        "$push": {
          "c": "$_id.c",
          "customerCount": {
            "$sum": "$customerCount"
          },
          "itemQuantity": {
            "$sum": "$itemQuantity"
          },
          "amount": {
            "$sum": "$amount"
          }
        }
      },
      dataKey: {
        $first: "$_id.c"
      }
    }

Below is the Java code for above snippet

Bson group = new Document("$group", new Document("_id", idDoc).append("data", new Document("$push", pushDoc)).append("dataKey", new Document("$first", "$_id."+orderBy)));

Exception is as shown below.

"com.mongodb.MongoCommandException: Command failed with error 168 (InvalidPipelineOperator): 'Unrecognized expression '$push'' on server localhost:27017. The full response is { \"ok\" : 0.0, \"errmsg\" : \"Unrecognized expression '$push'\", \"code\" : 168, \"codeName\" : \"InvalidPipelineOperator\" }\r\n\tat com.mongodb.internal.connection.ProtocolHelper.getCommandFailureException(ProtocolHelper.java:179)