cURL response invalid JSON message

I am using curl in c to send POST and GET requests. I have a custom callback for the responses.

static size_t callback(void *data, size_t size, size_t nmemb, void *userp)
{
    printf("This is the data:\n%s\n", data);

    jobj = json_tokener_parse(data);

    return size * nmemb;
}

However when I print the data, an extra 0 is appended. The first response I get:

{
  "success": true,
  "message": "You are now logged in as Tour Driver",
  "data": {
    "token": "137|jzGCVhGaaaNmsshAo29MktQZ9XMWXcxUpZw0jPUj",
    "user": {
      "id": "8cdbef45-fd36-4d29-8742-7fe537aabb57",
      "name": "Tour Driver",
      "roles": [
        "driver"
      ],
      "role": "driver",
      "profile_photo_url": "https:\/\/ui-avatars.com\/api\/?name=T+D&color=7F9CF5&background=EBF4FF",
      "teams": [
        {
          "id": "9bc3xh6d-ca2d-4af3-8f26-99376effa488",
          "name": "XAD IT",
          "membership": {
            "user_id": "8cdbef45-fd36-4d29-8742-7fe537aabb57",
            "team_id": "9bc3xh6d-ca2d-4af3-8f26-99376effa488",
            "role": "driver",
            "created_at": "2022-08-30T06:31:18.000000Z",
            "updated_at": "2022-08-30T06:32:36.000000Z"
          }
        }
      ]
    }
  }
}
0

Not sure why it is adding the zero at the end, but it can still be parsed to a JSON message.

However, when I try to do a GET request after that, the response is:

{
   "success":true,
   "message":"Found 1 trips",
   "data":[
      {
         "id":"8d030886-bf23-40b4-a6ab-5ae93a6c6f3a",
         "legId":null,
         "departureTime":"2022-09-14 13:34:00",
         "estimatedDepartureTime":null,
         "arrivalTime":"2022-09-14 13:46:00",
         "estimatedArrivalTime":null,
         "travelerReferenceNumbers":[
            
         ],
         "distance":6315,
         "customer":{
            "id":"e156cfa5-7a0f-4707-8294-695ae4f58a57",
            "email":"empty@test.com",
            "birthDate":null,
            "phones":[
               
            ]
         },
         "from":{
            "id":"94adb642-295c-456e-b019-edb6a8c77ec4",
            "name":null,
            "stop_reference_id":null,
            "stationId":null,
            "coordinate_id":"c581c9cd-0b04-4c9e-b695-552f4f8a10d0",
            "address_id":"a72f3071-84b1-4c7d-afc7-2552d29d40e8",
            "extraInfo":null,
            "coordinates":{
               "id":"c581c9cd-0b04-4c9e-b695-552f4f8a10d0",
               "lng":5.895355,
               "lat":51.959115
            },
            "physical_address":{
               "id":"a72f3071-84b1-4c7d-afc7-2552d29d40e8",
               "street":null,
               "houseNumber":null,
               "houseNumberAddition":null,
               "addressAdditionalInfo":null,
               "postalCode":null
            }
         },
         "to":{
            "id":"0eefc767-ac7a-49f5-917d-e519a959fd7b",
            "name":null,
            "stop_reference_id":null,
            "stationId":null,
            "coordinate_id":"1abc25cb-27be-47bd-8951-ade611fa01dc",
            "address_id":"3341b1ec-7215-4cab-8684-8f436a0801a9",
            "extraInfo":null,
            "coordinates":{
               "id":"1abc25cb-27be-47bd-8951-ade611fa01dc",
               "lng":5.942919,
               "lat":51.987601
            },
            "physical_address":{
               "id":"3341b1ec-7215-4cab-8684-8f436a0801a9",
               "street":null,
               "houseNumber":null,
               "houseNumberAddition":null,
               "addressAdditionalInfo":null,
               "postalCode":null
            }
         },
         "asset_type":null,
         "asset":null,
         "pricing":null,
         "suboperator":null,
         "state":{
            "id":"38d02ace-324d-46c2-9f36-f7fb5dd55999",
            "state":"NOT_STARTED"
         }
      }
   ]
1
}
0

Because of the 1 at the bottom, it is not parsable into a JSON message.

Whenever I don't add my own callback (but instead, let curl handle the callback) there is no 0 at the end.
I can't seem to figure out why the 0 and 1 are added to the message. Especially the 1, because it makes the JSON message invalid.

Any ideas?