how do I update a key field in a existing JSON string

This code is used to get to a JSON string an change the asignee field to the user currently logged in.

This code does not work because the key Full Name is not updating to the parameter Asignee. Can anybody tell me why its not working?

public void UpdateJsonAsign(String JSONstring, String Asignee){
    JSONObject obj = new JSONObject(JSONstring);
    obj.put("Full Name", Asignee);
}

via Chebli Mohamed