A surplus comma is output if the first field of a serializable class is tranient.

class JsonBug {
  static Void main(Str[] args) {
    json := util::JsonOutStream.writeJsonToStr(Thing())
    echo(json)
  }
}

@Serializable
class Thing {
  @Transient
  Int wot := 6
  Int sit := 9
}

gives

{,
"sit":9}

Okay, this is a dup of topic 2042 which never made it to ticket status.

Looks to be fixed in 1.0.64. I should really downgrade to a 32 bit machine sometime so I can use it!