How to modify Json

[Robokishan]

    char QC_JSON[1024] = <normalJson>;
    DynamicJsonBuffer jsonBuffer;
    JsonObject& root = jsonBuffer.parseObject(QC_JSON);
    if(root.success())
    {
      Serial.print("QC:");Serial.println(sizeof(QC_JSON));
      int lab = root["lab"];
      Serial.print("Json:");Serial.println(lab);
      memset(QC_JSON, 0, sizeof(QC_JSON));
      root.prettyPrintTo(Serial); 
      root.add("lab",int(3));      
      root.printTo(QC_JSON,sizeof(QC_JSON));
      String buffer = String(QC_JSON);
      Serial.println(buffer);
    }

now at this line

memset(QC_JSON, 0, sizeof(QC_JSON));
root.prettyPrintTo(Serial); 

output

{
  "": 2,
  "": "",
  "": "",
  "": ""
}

all the key values are gone why is this happening ?? Is root object address point to char buffer ?