Error: Expected BEGIN_OBJECT but was STRING at line 1 column 1, but data posted to server

Question

I'm getting this exception below, but the data is POSTed successfully.

Error: Expected BEGIN_OBJECT but was STRING at line 1 column 1

Can anybody tell me what the issue is in this format?

{
   "trip_id":"9",
   "user":"admin",
   "vehicle_no":"Hr26Eu6341",
   "trip_action":" Start",
   "tripdetails":[
     {
         "item_name":"Tyre",
        "item_position":"Axel1-left",
       "tag_id":"E123dervceEp32432425"
      },
      {
        "item_name":"Tyre",
        "item_position":"Select position",
     "tag_id":"E123dervceEp32432424"
      },
       {
         "item_name":"Battery",
         "item_position":"",
         "tag_id":"E123dervceEp32432424"
      },
      {
        "item_name":"Battery",
         "item_position":"",
         "tag_id":"E123dervceEp3243222"
      }
   ]
}