Can't setOrReplace fields on root level of DocumentMutation #18

Hi,
I'm using ojai:1.1-mapr-1703 in Scala and I can't force this method from DocumentMutation class:

DocumentMutation setOrReplace(String var1, Map<String, ? extends Object> var2)

to update fields on the root level of the document. For instance using it like this doesn't work:

val updateDocument = MapRDB.newMutation()
val docs = <...  a java.util.Map[String, Any] ... >
updateDocument.setOrReplace("", docs)
... update of the database with updateDocument ...

What I get in result are rows like this:

{
  "_id": "1",
  "field_name1": "aaa",
  "field_name2": {
    "$numberLong": 111
  },
  "creationdate": {
    "$date": "2019-02-07T13:05:20.101Z"
  },
  "": {
    "field_name1": "bbb",
    "field_name2": {
      "$numberLong": 222
    }
  },
  "lastmodificationdate": {
    "$date": "2019-02-07T13:07:04.060Z"
  }
}

Note the "": {...} field in the above JSON. In fact, I wanted the updated row to look like this:

{
  "_id": "1",
  "creationdate": {
    "$date": "2019-02-07T13:05:20.101Z"
  },
  "field_name1": "bbb",
  "field_name2": {
    "$numberLong": 222
  },
  "lastmodificationdate": {
    "$date": "2019-02-07T13:07:04.060Z"
  }
}

Changing "" to "new FieldPath.EMPTY" doesn't help either. I looked all over FieldPath documentation and couldn't find any alias for the root level of the JSON - does anything like this even exist?
Any help would be appreciated!