3rd-Eden memcached append json data

I am running a local instance of memcached and using 3rd-Eden memcached library to interface with it. The cache is used to store some JSON data, but sometimes I want to append new fields to the objects, as only the needed information is pushed to the memory, so what I do is creating a new JSON object containing the new fields, say:

var obj = {
    bio: "blah"
}

and then call the memcached.append(key, obj, callback_here). The problem is that this object is concatenated to the previous one, invalidating the JSON, so when I try to retrieve the stored data, I get the following error message:

[{"service":"service1","id":"abc"},{"service":"service2","id":"zed"}]}{"bio":"bl
                                                                  ^
SyntaxError: Unexpected token {

I know that I can re-upload the whole new object, but this way seems to be more efficient.