Problem of JSON Fields in Plugin Response-Transformer

So I have been trying this plugin by reading this post: https://docs.konghq.com/hub/kong-inc/response-transformer/#examples
The problem I encountered is that I want to transform the response to something like {“code”: -1} and I tried “config.add.json”:[“code:-1”], but the response becomes {“code”, “-1”}, -1 turning out to be a string.
Does anyone have any idea on how to fix this?