It looks like you are treating this JSON as a JSON array and are trying to add a value, I believe your syntax is incorrect.
You are trying to add the value "Mix" to array row 12345
As Vladimir points out with an array you probably would use Push, if it is just a regular object, you would create a 'node' {key:value} and add that to your JSON object.
Since I do all my JSON using google ... I recommend you google it 🙂