How to Get List<> result in json format

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MVC

Knockout.js

I needs to get a List<users> in Json format
I have tried the following
My KnockoutJS as

var self = this;
self.http = new httpWrapper();
self.GetAllUserForApproval = function () {
    self.http.post('/Lending/Account/GetAllUsersForApproval')
            .done(function (data) {
                alert(data.Payload);
            });
};

My Controller As

[HttpPost]
public JsonResult GetAllUsersForApproval()
{
    return Json(AccountRepo.GetUserForApproval());
}

The AccountRepo.GetUserForApproval() definition is like

public List<User> GetUserForApproval()
{
    return Context.Users
        .Where(x => x.IsConfirmationPending).ToList<User>() ?? new List<User>();
}

The Problem is when executing the self.http.post('/Lending/Account/GetAllUsersForApproval')
it shows the error as

Failed to load resource: the server responded with a status of 500 (Internal Server Error),

But if i change the AccountRepo.GetUserForApproval() definition to

public User GetUserForApproval()
        {
            return Context.Users
                .Where(x => x.IsConfirmationPending).FirstOrDefault() ?? new User();
        }

it works fine and i got the result in js success function ,
But i want List of users
Plz help me..