Ajax JQuery give me an error on Update Record pamater query which not initialized

I'm encountering an error when trying to update a record in ASP.NET Core.

Microsoft.Data.SqlClient.SqlException: 'The parameterized query '(@EmployeeID int,@FirstName nvarchar(4000),@LastName nvarchar(40' expects the parameter '@FirstName', which was not supplied.'

public class Employee
{
    public int EmployeeID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string City { get; set; }
    public int? ReportsTo { get; set; }
}

public void Update(Employee employee)
{
    string str = Configuration.GetConnectionString("dbConnection");
    using (SqlConnection con = new SqlConnection())
    {
        con.ConnectionString = str;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.Connection = con;
            cmd.CommandText = "UPDATE Employees SET LastName=@LastName, FirstName=@FirstName,City=@City WHERE EmployeeID=@EmployeeID";
            cmd.Parameters.AddWithValue("@EmployeeID", employee.EmployeeID);
            cmd.Parameters.AddWithValue("@FirstName", employee.FirstName);
            cmd.Parameters.AddWithValue("@LastName", employee.LastName);
            cmd.Parameters.AddWithValue("@City", employee.City);
            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
        }
    }
}

public ActionResult UpdateEmployee(int? id)
{
    return View(GetEmployees().Find(e => e.EmployeeID == id));
}

[HttpPost]
public JsonResult UpdateEmployee(Employee emp)
{
    Update(emp);
    return Json("Records updated successfully.");
}

@model CrudWithADONetAjaxjQuery.Models.Employee
@{
    ViewBag.Title = "Update Employee";
}
<h2>Update Employee</h2>
<script src="~/Scripts/jquery-1.10.2.min.js"></script>
<script src="~/Scripts/jquery.validate.min.js"></script>
<script src="~/Scripts/jquery.validate.unobtrusive.min.js"></script>
<script>
    $(document).ready(function () {
        $("#btnUpdate").click(function () {
            var employee =
                {
                    EmployeeID: $("#hfEmployeeID").val(),
                    FirstName: $("#FirstName").val(),
                    LastName: $("#LastName").val(),
                    City: $("#City").val()
                };
            $.ajax({
                type: "POST", URL: "/Home/UpdateEmployee", dataType: "json", contentType: "application/json",
                data: JSON.stringify({ emp: employee }),
                success: function (response) { window.location.href = "/Home/AddEmployee"; },
                error: function (response) { alert(response.responseText); }
            });
        });
    });
</script>
<div class="form-horizontal">
    <h4></h4>
    <hr />
    @Html.ValidationSummary(true, "", new { @class = "text-danger" })
    @Html.HiddenFor(model => model.EmployeeID, new { @id = "hfEmployeeID" })

    <div class="form-group">
        @Html.LabelFor(model => model.FirstName, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.FirstName, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.FirstName, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.LastName, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.LastName, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.LastName, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.City, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.City, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.City, "", new { @class = "text-danger" })
        </div>
    </div>
    <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <input type="submit" id="btnUpdate" value="Update" class="btn btn-default" />
        </div>
    </div>
</div>