Parse.com > Query Constraints > based on user Pointer

I have a class called AccountData and I would like to return all rows that relate to a particular user. In the class I have a Pointer to the User table which contains their "ObjectId"

I have tried with the following call to the API:

string url = "https://api.parse.com/1/classes/AccountData?" + WebUtility.UrlEncode("where={\"user\":\"fvJ8jPjyjx\"}");

where the fvJ8jPjyjx is the ObjectId of the user I want rows relating to...

The api doesn't throw any errors just returns:

{"results":[]}

I have also tried it using a "User Object" as follows:

public class AccountDataUser
{
    public string __type { get; set; }
    public string className { get; set; }
    public string objectId { get; set; }
}

building the object as follows:

AccountDataUser user = new AccountDataUser();
user.__type = "Pointer";
user.className = "_User";
user.objectId = objectId;
string jsonUser = JsonConvert.SerializeObject(user);

but this throws an api error.

Can anyone help me return the rows relating to a "user" please?

Thanks

UPDATE

Based on Ryans feedback I have reverted to trying to send an object...

This is what is being sent:

GET https://api.parse.com/1/classes/AccountData?where%3D%7B%22user%22%3A%22%7B%22__type%22%3A%22Pointer%22%2C%22className%22%3A%22_User%22%2C%22objectId%22%3A%22fvJ8jPjyjx%22%7D%22%7D HTTP/1.1
Content-Type: application/json
X-Parse-Application-Id: xxxxx
X-Parse-REST-API-Key: xxxxxx
Host: api.parse.com
Connection: Keep-Alive

The url is built with this line of code:

ParseModel.AccountDataUser user = new ParseModel.AccountDataUser();
user.__type = "Pointer";
user.className = "_User";
user.objectId = objectId;

string jsonUser = JsonConvert.SerializeObject(user);

string url = "https://api.parse.com/1/classes/AccountData?" + WebUtility.UrlEncode("where={\"user\":\"" + jsonUser + "\"}"); // this doesn't work

And the error I receive from the API is:

{"code":107,"error":"invalid json: {\"user\":\"{\"__type\":\"Pointer\",\"className\":\"_User\",\"objectId\":\"fvJ8jPjyjx\"}\"}"}