value cannot be null. parameter name string

Question

hi

 

I am stuck in this error "value cannot be null. parameter name string"

protected void btnupdate_Click(object sender, EventArgs e)
{
if (Page.IsValid)
{
using (var context = new schoolDbContext())
{
var UserName = txtUserName.Text;
var id = Int32.Parse(Request.QueryString["Id"]); // error in this line
var std = (from x in context.Users
where x.Id == id
select x).First();
if (std != null)
{
std.UserName = txtUserName.Text;
std.FirstName = txtFirstName.Text;
std.LastName = txtLastname.Text;
std.ClassAssigned = txtClassAssigned.Text;
std.Qualification = txtQualification.Text;
std.UserType = ddlType.SelectedValue;
std.Email = txtEmail.Text;
std.ContactNo = txtContact.Text;
context.SaveChanges();
}
}
}
}